THERE IS A mysterious alchemy inherent in the combination of milk and eggs. Whip them, stir them, add a little more of one, a little less of the other, and any cook can create a souffle', a cre me anglaise--or a flan.

Judith Salgado, who makes and sells flans for Judith Rose Desserts, knows from experience how simple flans are to concoct. "They are so easy and predictable," she says, "but it is essential that the ingredients be proportionately correct. Too much egg or milk can radically alter the consistency."

But even Salgado had to call Puerto Rico recently to consult her mother-in-law about a recipe for pumpkin flan, since "vegetable flans are not in my usual line of custards," she says.

As Salgado gently slips the newly created dish from its mold, the amber medallion appears to float on the plate, promising a wonderful side dish or light luncheon entre'e when served with a sauce.

Vegetable flans are custards--not sweet custards, like the flans served for dessert in Spanish restaurants, but savory custards. They are nothing more than pure'ed vegetables, such as pumpkin, spinach, eggplant or whatever, mixed with eggs, milk and spices. They can even be made the night before a dinner party, stored in the refrigerator, and reheated in a water bath 20 minutes before serving.

But tripping the light flantastic always seems to worry people about tripping over some technical matter. Perhaps this is the reason flans have not flourished, but are just beginning to surface as a delightful change from the more conventional vegetable dishes.

Richard Grausman, a Cordon Bleu chef who studied in Paris and teaches in New York, agrees that flans are frightening to many people.

"There is a false mystique that surrounds flans and baked custards that has been promoted by people who write about the difficulty in serving them. And if people do try to make them and fail because of such problems as runny custard or separation, they are put off by their failure. I think that most people never try to make them because it appears too difficult to achieve a proper texture. But all of this is overblown. They are simple to make and provide a wealth of variety."

Yannick Cam, owner and chef of Le Pavillon restaurant, recently reopened at its new Connecticut and L Street location, suggests that the delicate texture of flans is not yet appreciated. Because they are so intense in color and flavor, they are offered in small portions, and here, says Cam, is another problem for Americans, especially American men.

"The American man is very complex about this delicate food. When a man comes in with his wife, he will order it, but if he comes in with all men in a business party, he won't. It's a matter of education," says Cam.

The best place to start is by serving good flan, of course.

Both Grausman and Cam offer suggestions:

* Be sure the proportion of eggs to milk is correct. Too many eggs will reduce the flavor of the custard and make it tough or cause it to split and separate. Too much egg white will dry the custard.

* Never fill the baking mold more than three-quarters full.

* Do not allow the water bath (bain marie) that the custards are sitting in to come to a boil, or the custard will boil and pit as bubbles rise to the surface.

* Place the water bath at the same level as the custard.

Do not allow the water bath to spill into the filled molds.

Allow the custards to cool and set before unmolding.

Cam offers another tip to advanced flanmakers. He points out that the customary way to tell if the flan is done is to insert a knife in the center and pull it out; if the knife comes up clean, the flan is done. But Cam's technique is more subtle:

"I stick the knife in and then I touch the tip above and below my lips. If it is warm, the flan is done." JUDITH ROSE'S PUMPKIN FLANTASTIC (Makes 6 to 8) Butter for greasing baking molds plus 2 tablespoons unsalted butter 3 eggs 9 tablespoons sugar 3/4 cup evaporated milk 1 pound canned pumpkin 1 teaspoon cinnamon 1/2 teaspoon nutmeg 1/2 teaspoon ground ginger Pinch of cloves 1 teaspoon finely grated orange rind 2 tablespoons heavy cream 2 tablespoons rum 1/3 cup chopped walnuts (reserve for topping)

Generously butter 6 to 8 baking molds ( 3/4- to 1-cup capacity souffle' dishes or ramekins). Melt the 2 tablespoons butter and set aside.

Using a food processor fitted with a metal blade beat the eggs and sugar together. Add the milk, pumpkin, melted butter, orange rind and spices to the egg mixture and blend thoroughly. Blend in the heavy cream and rum. Pour the pumpkin mixture into the buttered baking molds.

Place the molds in a pan and pour enough boiling water around the molds so that the water comes up the sides and sits level with the pumpkin mixture. Be careful not to splash any water into the molds.

Transfer the water bath to a 350-degree oven and bake 35 minutes or until the pumpkin mixture feels firm. Remove the dishes from the water bath and let them sit at least 10 minutes before unmolding.

Gently loosen their edges with a knife, invert to unmold and sprinkle the chopped walnuts on top of the flans and serve. Pumpkin flan is wonderful with ham and poultry. CARROT FLAN (Makes 6)

You can make these flans a day or two in advance and reheat in a water bath on top of the stove for 30 minutes. Butter for greasing baking molds plus 2 tablespoons unsalted butter, at room temperature 7 medium carrots (14 ounces total), peeled and cut into 1-inch pieces 2 large eggs 1/2 cup milk 3 tablespoons heavy cream 1/2 teaspoon freshly grated nutmeg 1/2 teaspoon salt Freshly ground black pepper Crushed chervil for garnish

Butter six 1/2-cup baking molds. Boil the carrots until very soft, about 30 minutes, and drain them.

In a food processor fitted with the metal blade, process the carrots and butter for 10 seconds or until pure'ed, stopping once to scrape the bowl. (You should get 1 cup of pure'e.)

Add the remaining ingredients, except the chervil, and process for 30 seconds. (You should get about 2 1/2 cups in all.) Add pepper to taste and pour the mixture into the molds.

Place the molds in a pan and pour enough boiling water around the molds so that the water comes up the sides and sits level with the carrot mixture. Bake at 375 degrees for 30 to 35 minutes or until the mixture feels firm.

Remove the dishes from the water bath and let them sit at least 10 minutes before unmolding. Gently loosen their edges with a knife, invert to unmold and spoon the lemon sauce (see recipe below) on top of the flans and sprinkle with chervil.

Variations--Broccoli flan: Substitute 14 ounces of broccoli (stems and florets), cooked until tender. Season with 1 teaspoon salt, freshly ground pepper and freshly ground nutmeg to taste. Proceed as in the carrot flan and bake for 30 minutes at 375 degrees or until the mixture is firm. Spoon over lemon sauce (see recipe below).

Cauliflower flan: Substitute 18 ounces of cauliflower, cooked until tender, and 3 ounces of gruyere cheese, shredded. Season with 1 teaspoon of salt; freshly ground pepper and freshly ground nutmeg to taste. Proceed as in the carrot flan, baking at 375 degrees for 35 minutes or until the mixture is firm. Spoon over lemon sauce (see recipe below). From "The Pleasures of Cooking" VERY LEMON SAUCE (Enough for 6 flans) 1/2 cup lemon juice 4 large egg yolks 3 tablespoons heavy cream 1 1/2 sticks unsalted butter, melted Salt and freshly ground pepper to taste

Place the lemon juice and egg yolks in the top of a double boiler. Whisk them together over moderate heat until the yolks begin to thicken. Beat in the heavy cream 1 tablespoon at a time. (The sauce will become very thick and creamy.)

Immediately remove the pan from the heat and gradually whisk in the melted butter, 1/2 tablespoon at a time, until the sauce is very thick.

Add salt and pepper to taste; spoon the sauce over the flans and sprinkle with the crushed chervil. YANNICK CAM'S FLAN D'AIL DOUX AU CERFEUIL, CHAMPIGNONS SAUVAGES, FONDUES DE TOMATES AU BASILIC (GARLIC AND MUSHROOM FLAN WITH TOMATO AND BASIL SAUCE) (Makes 8) 1/2 pound garlic cloves 9 tablespoons unsalted butter 1 large shallot, minced 8 to 9 large white mushrooms, sliced very thin 1 tablespoon heavy cream 8 egg yolks 2 egg whites 2/3 cup milk 1/4 teaspoon crushed chervil Salt and freshly ground black pepper to taste Sauce: 1 tablespoon olive oil 1 very ripe tomato, cored and chopped very fine 1 shallot, minced 1 garlic clove, crushed 4 to 5 fresh basil leaves, chopped (1 tablespoon dry crushed leaves) Salt and pepper to taste

Blanch the garlic cloves in boiling salted water for 3 minutes to remove the bitterness, drop them into cold water, drain and peel.

Melt 4 1/2 tablespoons of the butter in a saucepan and very slowly saute' the cloves until they are blond in color and completely tender, about 3 to 5 minutes. Place onto a plate to cool.

Melt the remaining butter and saute' the shallot and mushrooms. In a food processor, pure'e the garlic, shallot and mushrooms and put the mixture through a food mill to make an even finer paste. In the food processor, combine the cream, egg yolks, egg whites, milk and chervil. Add the garlic paste and process for 30 seconds.

Pour the mixture into 8 buttered 1/2-cup capacity baking molds, souffle' dishes or ramekins.

Place the molds in a pan and pour enough boiling water around the molds so that the water comes up the sides and sits level with the garlic mixture. Bake at 375 degrees for 30 minutes or until the mixture feels firm.

Remove the dishes from the water bath and let them sit at least 10 minutes before unmolding (at least 20 minutes if using larger molds). Gently loosen their edges with a knife, invert to unmold, spoon the basil sauce on top.

To prepare the sauce, heat the olive oil over moderate heat. Saute' all of the ingredients together for 3 minutes and spoon onto the flans.