THE GLORIES of serendipity aside, there are times when you just have to sacrifice a Saturday afternoon for some future Saturday night fervor.

With a minimum of effort and expense, you can in a few hours put up a couple of jars of pepper jelly for hors d'oeuvres, a crock of homemade chutney, and freeze enough fish stock, vegetable soup and spaghetti sauce to disarm any guest. There is nothing more sophisticated than saying, "Let's run by the wharf and I'll whip up some bouillabaisee."

The only rule here is flexibility. Use what ingredients are on hand.

Learn to collect meat trimmings and bones in one freezer bag and vegetable tops, peelings and such in another. (Stock made from vegetable trimmings is not only far more delicious than water or boullion, it makes use of all those extra vitamins.) If you use bottled pearl onions in coq au vin or boeuf bourguignonne, save the water. Save any such delicately flavored juisces from bottled baby carrots or whole tomatoes, and last spashes of V-8 or clam or clamato juices.

If you squeeze lemons, save the used fruit. Let your friends know you can make use of bones - hundreds of beautiful legs and ribs are tossed out every night. A country ham bone lends an incomparable smoky scent and flavor to chili or soup as well as beans. (Once you get into this pre-cooking habit, you'll discover that rendered country ham fat makes the perfect cornbread.) And never throw out the necks or backs or innards of fowl, or even the seemingly dried-out and picked-over carcass of a baked hen.

There is nothing simpler than homemade vegetable soup, but making it has become a lost art. There is not "recipe" per se - in fact, there are no recipies for any of these dishes. The ingredients depend on the season and on the whim of the creator.

Start with the biggest pot in the house, the pasta or clam pot, perhaps. Put in all the stashed-away bones and veggies from the freezer, plus three or four shakes of a big salt cellar, and cover with water. Or, if the freezer is bare, ask the butcher for some big stock bones and have him cut them up so the marrow is exposed.

Bring to a boil and let simmer for an hour, occasionally skimming. Go ahead and start cutting up the vegetables and add to the stock any peelings, or stems. (Use your common sense; such vegetables as green peppers are too bitter.) Let the stock cool a little and then strain, squeezing the vegetables over a sieve. Cheesecloth is nice for a final strain to eliminate any grit.

Now start adding the vegetables. Don't worry too much about proportions at first; since everyone has a different idea about how thick or thin soup ought to be, you can't go wrong.

Use any vegetable you like. The longer-cookers go first - meat if you want it, onions, celery and carrots; and throw in the canned tomatoes (chopped and in their juice). Give that bunch a 1/2-hour head start, then add potatoes (you can leave the scrubbed skin on), turnips or parsnips. Last come squash, cabbage, cauliflower, broccoli, corn (frozen is perfectly fine$&(WORD ILLEGIBLE $&)See COMPANY, N6, Col. 4Let it all simmer together for a final 15 to 20 minutes, cool and ladle into containers and freeze.

You can alter the flavor of this toward the minestrone side by exchanging some of the vegetables for noodles or pasta, green beans, chick peas and the like.

Incidentally, plastic-lidded coffee cans are perfect for freezing, and produce two hefty servings per one-pound can when defrosted. If the inside gets rusty with use, a quick scrub with steel-wool or other abrasive cloth will rehabilitate it easily.

For fish stock, simmer no more than 15 minutes in 3 cups mixed water and white wine or vermouth: a couple of pounds of fish heads and trimmings (ask the fishmonger - he'll usually give them away); a handful of chopped onion or scraps of onion skins; a chunked carrot and celery stalk; pepper; parsley and/or basil; a lemon rind. Shells from shrimp, crab or lobster are a terrific extra (haven't you always felt guilty about wasting all those crab legs?). Skim the stock, and at the last minute add any oyster liquor, clam juice, or such. Strain stock and freeze.

You can use this stock as the base for sauces for seafood as well as for soup. Throw in some chopped plum tomatoes, mixed fish, mussels, lobster meat or shrimp or what-have-you and produce a quickie bouillabaisse. Or cubed potates, onions, minced clams (canned will do fine) and cream for chowder.

Both the chutney and the pepper jelly make perfect gifts, so set a few small jars aside to take with you instead of wine when you go out to dinner.

The chutney will take a long while to cook, so if you're doing a coupl of these projects simultaneously, start here. Chop 16 cups of pears (seeded but not peeled), 4 lemons and 4 or more cloves garlic. Add 5 1/2 cups of brown sugar and 4 cups of apple cider vinegar; 4 tablespoons ground ginger, 3 teaspoons salt and 1/2 teaspoon cayenne. Cook until transparent. If you like, add a jar of small candied ginger pieces. Store in the refrigerator in stoneware crocks or in sealed and sterilized Mason jars.

For pepper jelly, chop in blender (in small portions) 1/4 to 1/2 cup hot peppers (red or green) and 1 1/2 cups sweet bell peppers in 1/2 cup of vinegar. Combine in large pot with 6 1/2 cups of sugar. Bring to boil and boil 3 minutes. Add 1 bottle pectin and boil 1 minute more. Remove from heat and let sit 5 minutes. Strain if desired. Serve with cream cheese and crackers as an hors d'oeuvre or with strong meats such as lamb.

You'll have to experiment with homemade spaghetti sauces and chilis to find the mixture you want. As a beginning for spaghetti sauce, brown a pound or so of lean ground beef in a heavily-salted skillet; lift the browned meat out with a slotted spoon and put in a pot (an iron kett e is good). Brown 2 or 3 large chopped onions (more than you think!), a large chopped green pepper and several cloves of garlic in the beef fat until soft. Add to pot. (If you have saved mushroom stems, mince them and brown them with the onions).

Add 2 large cans of good Italian tomatoes chopped, a large jar of some "extra-thick" prepared sauce, parsley, basil, oregano and pepper to taste and 1/2 cup or more red jug wine. Simmer 20 to 30 minutes and freeze. To refresh flavor after long freezing, splash a little more wine in and simmer a few minutes.

For a red clam sauce, cook chopped plum tomatoes, sauteed onions and bell peppers, minced garlic and herbs in (one or a mixture of) tomato juice, vermouth and clam juice until thick, then add canned minced clams and simmer a few minutes more. CAPTION: Illustration, no caption