"In summer it's impossible to cook," admits Richard Sax, author of "Cooking Great Meals Every Day," (Random House, $15.95). "Yet summer makes me want to cook more than any other season. I look forward all year to the profusion of tomatoes, corn, fresh basil and blueberries, and they inspire me to entertain despite the heat."

Because he enjoys preparing meals with summer foods, Sax has evolved a modus operandi for creating simple, spontaneous, hot-weather menus. If a few friends call up unexpectedly in mid-afternoon and Sax decides to invite them over for dinner that evening, he can count on having something interesting in the house which can become the focus of the meal. Says Sax, "After I put down the phone, I take a look at what I have around, think about what looked especially good in the market the morning, and come up with a simple menu."

His first rule is that when you do turn on the burners, always prepare extra. For example, if you're poaching fish fillets for an evening meal, make double what you'll need. The next day you can serve cold poached fish with a mayonnaise flavored with chopped tomato and shredded basil. Or, if only one fillet is left over, flake it into a salad of celery, carrots and red onion, dress it with a vinaigrette, and serve it in a halved avocado. Alternatively, prepare a sauce for pasta by reducing some heavy cream, adding a chopped tomato and saute'ed onion, and--at the very last minute--the flaked fish and shredded dill or tarragon.

Sax also uses the "too much is better" philosophy when it comes to preparing sauces and dressings. When making mayonnaise, for example, he'll always whip up some extra, one day flavoring it with chopped red pepper and cucumber as a topping for cold chicken, another day blending in some sour cream and fresh garlic and tossing it with chopped raw vegetables.

"When entertaining in the summer," says Sax, "it's important to be flexible. Don't get locked into a set notion about what constitutes an appropriate main course. For me, a large bowl of cold Oriental noodles or a fish soup can be the focal point of a meal. I might add a tomato or rice salad, a nice loaf of bread, and fruit for dessert."

Another aspect of flexibility, according to Sax, is inventiveness. He finds that two foods that particularly invite improvisation are pasta and rice. He suggests cooking a large batch of either until just done (but still chewy) and running it under cold water to wash off the surface starch. Then toss the batch immediately in milk (if you are planning a dairy-based sauce) or oil to separate the strands or grains.

Develop a cold pasta or rice salad by first taking a look at what you have in the refrigerator. If, for example, you see some cold asparagus, a few slices of ham, a red pepper and a bunch of scallions, these may all be finely minced and tossed into the pasta or rice with a lemony vinaigrette.

When you are putting together the salad, Sax says, consider the color, flavor and texture of the ingredients. If you're using green pepper, then you probably won't want another green like beans or asparagus. For assertive flavor, Sax always selects a member of the onion family, and if he's aiming for crunch in the salad, he'll add finely minced celery, a nice contrast to the relatively soft rice or noodles.

Your choice of salad dressing will depend on the weather. If it's really hot, you wouldn't want a heavy mayonnaise or sour cream dressing, but might opt for a light vinaigrette, perhaps seasoned with a few fresh herbs. (The best way to store fresh herbs and watercress, Sax advises, is to set the roots or stems into a glass of water and cover the leaves with a plastic bag.)

Sax finds that by using herb butters, he can almost effortlessly transform a grilled fillet of fish into an elegant main course. A special favorite is fennel butter which he makes by spinning a stick of softened butter in the food processor with two tablespoons of fennel seed. He then melts the butter and lets it sit off the heat for an hour, adds a tablespoon of lemon juice and a dash of pernod and chills the butter until firm. Before it hardens entirely, he molds the butter into a log in plastic wrap and freezes it. "I lop off pats, sometimes straight from the freezer, to garnish fish," explains Sax.

For dessert, Sax likes to concentrate on fresh fruit. He might serve strawberries which have been hulled and "ripened" for an hour after they've been sprinkled lightly with sugar. Before serving, he splashes on some orange juice.

He often makes a fruit salad of sliced nectarines, plums and blueberries, tossing the fruits with a little sugar an hour before spooning them over ice cream or slices of pound cake. "Cannoli cream," says Sax, "prepared in about 30 seconds, makes a knockout dessert when spooned over fresh fruit."

All year around, but especially during the summer, Sax advocates having a light-hearted attitude towards food. "Recipes aren't gospel," he states. "Learn how to play with food and let it inspire you to invent new dishes with the summer's bounty." Ingredients to Have on Hand

For summer entertaining, Sax always keeps a good supply of the following on hand: Ripe tomatoes, parmesan cheese, fish stock frozen in 1-pint quantities (for poaching fish and making fish stews), white wine (for drinking and for poaching fish), lemons, onions, garlic, parsley, a variety of pastas and rice (Sax prefers Uncle Ben's as he finds the grains of cooked rice "separate beautifully").

Here is a selection of Richard Sax's favorite recipes for summer entertaining, all taken from his cookbook "Cooking Great Meals Every Day," written in collaboration with David Ricketts. CREAM OF CANTALOUPE SOUP (4 servings)

This light, unusual soup makes a refreshing opening to a hot-weather meal. It takes practically no time to prepare, since there's no cooking involved. Try varying this recipe by using other melons, and for additional appeal, garnish with thin slivers of prosciutto or fresh mint leaves. 2 ripe cantaloupes 3 tablespoons white wine 2 to 3 tablespoons fresh lemon and/or lime juice 1 1/2 tablespoons cointreau 2 to 3 tablespoons Coco Lopez or other canned sweetened coconut cream, or to taste 3 to 4 tablespoons heavy cream Salt, if desired Freshly grated nutmeg and freshly ground white pepper to taste

Cut the cantaloupes in half. Scoop out and discard the seeds and stringy material. Remove the flesh; pure'e in the food processor or blender until very smooth. Pour the pure'e into a bowl.

Add the wine, lemon juice, cointreau, Coco Lopez and heavy cream to the pure'e. Season with salt, if desired, nutmeg and white pepper; chill well. Serve in chilled soup bowls. RICE SALAD WITH PEAS, PROSCIUTTO AND BASIL (6 servings) Try this salad as stuffing for ripe tomatoes. 3 tablespoons unsalted butter 10-ounce package frozen small peas, partially thawed 1 1/4 cups long-grain white rice 6 ounces prosciutto, cut into slivers Salt Freshly ground black pepper 3/4 cup olive oil, or as needed 1/4 to 1/2 cup chopped fresh parsley Minced fresh basil Juice of 2 lemons

Heat the butter in a large skillet over high heat. When the foam subsides, add the peas. Toss until heated through, about 5 minutes. Reserve.

Cook the rice in a large pot of boiling salted water until al dente, about 12 minutes. Drain. Rinse under hot water. Drain again. You should have about 3 cups of cooked rice.

While the rice is still warm, combine in a large bowl with peas and prosciutto. Season with salt and pepper. Add enough oil to moisten the rice. Let the mixture stand at room temperature, covered, to allow the flavors to blend.

Just before serving, mix in the herbs and lemon juice. Correct seasonings. Toss well. PASTA WITH UNCOOKED TOMATOES AND MOZZARELLA (4 to 6 servings)

Sax was inspired to develop this recipe by Vincenzo Buonassisi, the Italian author of "Pasta" (Lyceum Books). The dish is a plate of hot noodles, a cool chunky tomato sauce and barely melted cheese. 1 pound ripe tomatoes 1/2 cup olive oil 1/4 cup fresh basil leaves, coarsely chopped 1/4 cup oil-cured black olives, pitted, cut into thin slivers 1/4 cup freshly grated parmesan cheese Salt and freshly ground black pepper 8 ounces mozzarella cheese, in 1 piece 1 pound pasta, such as conchiglie, shells or linguine

Peel and core the tomatoes. Halve them lengthwise; seed. Cut tomatoes into 1/4-inch strips. In a medium bowl, toss the tomatoes with oil. Add basil, olives, parmesan, salt and pepper; toss. Cover. Marinate, refrigerated, 2 hours or more. Just before serving, cut mozzarella in 3/8-inch cubes. Place in a large serving bowl.

Cook the pasta in a large pot of boiling salted water until al dente. Drain. Add to mozzarella; toss to lightly melt the cheese. Add marinated tomato mixture. Toss everything together. Serve immediately. POMMERY MUSTARD BUTTER

Garnish your next grilled steak with this delicious butter. 1 stick (4 ounces) unsalted butter, softened 1 1/2 tablespoons pommery (coarse-grained) mustard 1 teaspoon fresh lemon juice Salt and freshly ground pepper to taste

In a food processor, combine all of the ingredients into a smooth paste. Chill somewhat, then shape into a log in plastic wrap and freeze until needed. CANNOLI CREAM (Makes about 2 cups)

Sax learned this recipe from Johanne Killeen, a Rhode Island restaurateur, and it has become a staple on his summer menu. It takes only seconds to prepare, and can be refrigerated for two or three days. Serve bowls of cannoli cream with berries or other fresh fruit and let guests dip as they please. 15 ounces ricotta cheese 1/2 cup orange marmalade, or to taste 2 tablespoons cointreau, or other orange liqueur 1/2 teaspoon pure vanilla extract

1 1/2 tablespoons semi-sweet chocolate bits

In a food processor, blend the ricotta with on-off motion until smooth. Or whisk by hand. Do not use a blender. Add the marmalade, liqueur and vanilla to the ricotta; process very briefly, just until blended. Add the chocolate bits. Process briefly with on-off motion until most of the bits are chopped, but a few are left whole. Or chop the bits with a knife, then fold into the mixture without processing. Chill for an hour or so before serving. GRANITA DE CAFFE CON PANNA (Coffee Ice with Cream) (4 to 6 servings)

Italian espresso, broken up into coarse crystals as it freezes, then served in tall glasses with barely whipped cream, makes a leisurely way to linger over coffee and dessert, all in one. 3 cups very strong coffee, preferably espresso, hot 1/2 to 2/3 cups sugar, to taste 1/2 to 3/4 cup heavy cream, cold Sambuca or anisette to taste

Brew the coffee; add sugar while hot. Cool; chill thoroughly in the refrigerator. Pour the coffee into a shallow pan such as a pie plate or ice cube trays without dividers. Carefully place in the freezer about 2 1/2 hours before serving.

When mixture is frozen around the edges, about 45 minutes, use 2 knives to combine firm and liquid portions, cutting the mixture into coarse crystals. Return to the freezer. Repeat the blending and cutting 1 or 2 more times. The last time, the mixture should be quite firm throughout. Cut into large, even crystals; spoon into wine glasses. Whip the cream until just fluffy, but not too stiff (soft peaks). Top each serving with some of the cream. Serve immediately. Flavor the cream with a little anisette or sambuca, if you like.