IN THE BEGINNING there was Marty and Helen Blank's apartment. And at that beginning, that first Passover seder the Blanks held in Southwest Washington 10 years ago, Gary Jonas had to bring his own table and chairs from his apartment upstairs. He also brought Roz Levy, who later not only became a permanent member of the Passover celebration group but also his wife. In the annals of the Blanks' seders, it is recorded as "Roz formally married into the seder."

The story is repeated dozens--hundreds, thousands--of times in this transient city where everyone is from somewhere else, which means that everyone's family is somewhere else at holiday time, especially when a holiday comes midweek, as Passover does this year. Grandparents, aunts, uncles and cousins are likely to be substituted, as at the Blanks' decade of seders, with co-workers, college roommates, upstairs neighbors.

And they become a surrogate family tree. Marty Blank is a management consultant; Helen is with the Children's Defense Fund. Erica Dolgin, a Justice Department attorney, went to college with Helen Blank; Barbara and David Leavitt--art editor and attorney--were in high school with her. When blood relations are not around to constitute a family, we reach out to other kinds of relationships.

Then, having formed an improvised family, we sometimes bring our primary family into it. Now Helen's mother comes from Detroit. Lauren Weinberger, Helen's sister, attends the seders; her husband Alan married into the group two years ago. On Marty Blank's side are Gary and Roz Jonas, of University Research Corporation, and Gary's father, who comes down from New York.

Thus they become part of the tradition. The Blanks' seder usually has 20 to 30 at the table, and half of them are an unchanging nucleus. Individual members may not see each other more than once a year, but, as Margie Siegel, one of the original members, put it, "It is not a seder unless they are there." As in many families, not only are the roles defined, but people have a special place to sit at the table. For 10 years Roz Jonas has sat to the right of Marty Blank, who always leads the seder; for eight years Erica Dolgin has sat on his left. While some members have married into the seder, others have left when they divorced a member of the group. And, like any family, they see each other through troubled times; this year three members who last year had good jobs are unemployed.

As a tradition builds, so do its accoutrements. While the group depends on paper plates as much as necessary--since seder utensils can only be used during that holiday--people have contributed soup bowls and wine glasses, which the Blanks store in their home from year to year. The Blanks' supply of haggadahs--Passover prayer books--is supplemented by the Katz's, since they bought the same versions for their second-night seder in order to mesh with the Blanks' seder's needs.

The final step is the group reproducing itself, building another generation. Liza Blank, age 8, and Molly Blank, age 5, have not only "grown up at the seder table", says Roz Jonas, they have made the matzo cover, the Elijah's cup, a seder plate and a three-dimensional haggaddah which have become part of the ritual. They have also contributed their Hebrew school tunes to add to the confusion of musical traditions the members brought with them from both the East Coast and the Midwest. Every year there is a quibble about tunes and every year the members resolve to work on them in advance; like many family resolves, this one remains undone. But Marty Blank insists the singing at the seder has improved since his daughters have been attending Hebrew school. The quibbling may, of course, grow greater as other children bring their own Hebrew schools' variations. This year three more children are coming, and both Wendy Kahn and Lauren Weinberger are pregnant. The real test will come when the children grow up and move away from their seder "home."

All those mother-in-laws their spouses will have to live up to! The Blanks' children have grown up on Roz Jonas' haroset and stewed fruit. And Gary Jonas has been bringing his unusual matzo balls, made with whole matzos as well as matzo meal according to his mother's German recipe, since 1977. Barbara Leavitt's cold asparagus is such an integral part of the seder, it was considered sufficient crisis when she could not attend that the group invited her in-laws so that the seder could have the Leavitts and their asparagus. Nancy Amidei, who is not Jewish, has been attending for eight years, and is assigned something that requires no cooking--wine, matzo, jars of gefilte fish. Margie Siegel is the baker of the group; she experiments with dessert recipes, sometimes European tortes that don't require flour (having found that substituting matzo flour in cake-flour recipes does not work), other times Passover recipes from cookbooks published by synagogue sisterhoods ("If you skip the jello mold section . . ."), usually cutting down on the sugar. She has contributed a Passover version of lemon meringue pie plus an apple cake inherited from her stepmother. The year she was working 14 hours a day, though, Siegel brought the jars of gefilte fish. Completing the transition from family-oriented to office-oriented society, Roz Jonas got her sponge cake recipe from "an office Jewish mother."

And in this new social configuration there are unique crises, such as the time Erica Dolgin set her tureen of chicken soup on the roof of the car and a slammed door caused it to spill. As Roz Jonas recorded, "Five bouillon cubes took care of everything."

Like many families, this one has migrated, from the Blanks' Southwest apartment to their home in Chevy Chase. Yet only once, when the Blanks were remodeling their home ("though not for the exclusive purpose of accommodating the growing seder," cracked Roz Jonas), did the seder relocate to another household, the Jonas'. That year the customary ending of the seder service, "Next year in Jerusalem," was replaced with, "Next year at the Blanks'."

RICKI DOLGIN'S CHICKEN SOUP (24 servings) 2 roasting chickens or hens, cut up 6 carrots, cut in large chunks Handful of celery leaves and stems 2 large onions, cut in large chunks Several parsley stems and remains of other vegetables (broccoli, green beans) Extra chicken parts (necks, hearts, gizzards) as available 10 whole peppercorns 6 whole cloves

Place all ingredients in a large stockpot. Cover contents of pot with water. Bring water to boil. Skim off scum. Add salt to taste. Cover loosely, lower heat and simmer for 3 hours. Remove chicken and vegetables from pot, reserving bits of chicken and carrots for serving in soup; discard the rest. After stock is cool, pour through fine strainer. Refrigerate. When fat on the surface has solidified, remove it (save it for frying chicken). Serve soup, hot, with matzo balls, cooked carrots, bits of cooked chicken.

GARY JONAS' MATZO BALLS (Makes 12 to 15) 6 to 8 matzos 1 onion, finely chopped 1 teaspoon chopped parsley 2 tablespoons fat or oil (sometimes more) 3 to 4 eggs 1 teaspoon salt 1/2 teaspoon pepper 1/4 teaspoon nutmeg 6 to 8 tablespoons matzo meal

Soak matzos in water until softened. Squeeze to thoroughly remove water. Combined softened matzos with onions and parsley. Heat fat or oil in a large pan and add matzo mixture, heating and stirring until it is dry; add more fat if necessary. Cool mixture, and add eggs, salt, pepper and nutmeg. Add matzo meal, and roll in balls the size of a walnut. In rolling, add more matzo meal (put some on your hands) until the dough does not stick to your hands. Add matzo balls to boiling soup and simmer, covered, for 10 minutes.

ROZ LEVY JONAS' HAROSET 1 apple, peeled and chopped or coarsely grated 1/4 cup walnuts, finely chopped 1/2 teaspoon cinnamon 1/2 teaspoon sugar 1 tablespoon seder wine Honey to taste and blend

Mash apple in bowl with nuts, cinnamon, and sugar. When mixture is smooth, add wine and honey and mix again.

HELEN BLANK'S MOROCCAN CHICKEN (4 servings) 1/3 cup olive oil 1 large onion, thinly sliced 1 clove garlic, mashed 1 tablespoon chopped fresh parsley 1 tablespoon chopped coriander 1 teaspoon salt 1/2 teaspoon black pepper 1/8 teaspoon powdered saffron 1 chicken, cut up 1 lemon, cut in 8 wedges 1/2 cup green olives

Heat oil in large dutch oven. Stir in onion, garlic, parsley, coriander, salt, pepper and saffron. Add chicken and turn to coat with onion mixture. Arrange lemon wedges over top. Cover and simmer, turning occasionally, for 1 hour or until tender. Remove chicken to warm serving platter. Arrange lemon wedges on top and keep warm. Reduce liquid, stirring, over high heat until a thick sauce. Add olives and heat through. Pour sauce over chicken and serve.

MARGIE SIEGEL'S LEMON MERINGUE PIE (8 servings) Matzo meal crust: 1/4 cup vegetable shortening 2 tablespoons sugar 1/4 tsp salt 1 cup matzo meal 3 teaspoons water, approximately Grated lemon peel (optional)

Filling: 4 eggs, separated 1 1/4 cups sugar 1 tablespoon potato starch Dash of salt 2 tablespoons water 2 teaspoons grated lemon rind 6 tablespoons lemon juice

To make crust, cream shortening; add sugar and salt and mix well. Gradually work in the matzo meal. Add water, drop by drop, sufficient only to hold mixture together. Press into a 9- or 10-inch pie plate, shaping well into the bottom and sides. Bake in a 350-degree oven for about 10 minutes, or until lightly browned.

To prepare filling, beat the egg yolks slightly. Mix them with 1/2 cup of the sugar, the potato starch, salt, water, lemon rind and lemon juice. Cook over boiling water, stirring constantly, until the mixture is thick and smooth. Remove from heat. Prepare a meringue by beating the egg whites with the remaining 1/2 cup sugar. Beat the sugar in gradually, beating well after each addition. Fold about 1/2 of this meringue into the yolk mixture and turn into the baked matzo pie shell. Cover with remaining meringue, piling it on lightly, but covering all the edges. Bake in a 325-degree oven for about 15 minutes, or until lightly brown. If preferred, all of the meringue can be folded into the yolk mixture, and baked as above.

ROZ LEVY JONAS' SPONGE CAKE (12 servings) 7 eggs, at room temperature 1 1/4 cup sugar 1/2 lemon 1/4 cup matzo cake meal 1/2 cup potato starch

Line bottom of an ungreased 10-inch tube pan or springform with brown paper (which peels off after cake has cooled).

Separate eggs, putting yolks in smaller bowl. Grate 1/2 lemon (rind, juice, pulp) into yolks. Add sugar and whisk until light. Fold in meal and starch and blend well. Batter will be stiff but pourable. Beat whites until stiff and fold yolk mixture gently and thoroughly into whites. Bake for approximately 1 hour at 325 degrees until top is lightly brown but not dry. (This is a matter of trial and error.)

Invert immediately over narrow end of funnel or any other arrangement just so air gets under cake, and let sit inverted until cold. Remove cake by passing sharp knife around inside of pan and tube, and invert onto plate.

MARGIE SIEGEL'S APPLE CAKE (Makes an 8-inch cake) 3 eggs 3/4 cup sugar 3/4 cup matzo cake meal 1/3 cup peanut oil 5 apples, pared and sliced

Topping: 1/3 cup walnuts 1/2 cup sugar 2 teaspoons cinnamon

Mix the eggs, sugar and cake meal together. Add oil. Mix well. Pour half the mixture into a lightly greased 8- or 9-inch square baking pan. Spread a layer of apples over the batter. (If desired, add mixture of sugar/cinnamon, white raisins and nuts on top of apple layer.) Pour the remaining batter into the pan and cover with the rest of the apples. Sprinkle the topping over the apples. Bake 1 1/2 hours at 350 degrees. To make a larger cake, double the recipe and place in a 9-by-13-inch oblong pan.