THIS HOT-weather meal has both a hint of fall and the grace to rise above yet another summer night. It begins with an elegant iced watercress soup whose green freshness is enlivened by whipped cream, which makes lovely swirls in the bowl. Watercress now is full of flavor, bite and the promise of cooler weather.

Next are small rainbow trout, baked quickly and then masked with a white wine and cream reduction, a delicate sauce worthy of this subtle fish. The trout are accompanied by tiny new potatoes and a beautiful salad made up of three shades of red -- beets, baked like potatoes to keep the flavor intense and the texture firm, red onions and wonderful fresh tomatoes. Dessert is a bowl of textured summer fruits, some lightly stewed and others fresh, macerated in syrup and plenty of kirsch.

This watercress soup would be appropriate for the most important dinner party, served iced now or hot later on. The watercress, wilted in the flour-butter mixture, retains its bright color, freshness and flavor. Despite the additon of flour, the soup is not thick on the tongue but rather gains from the smallest bit of underbody. Because condensed chicken broth is used as the liquid, little (if any) salt is needed, so taste carefully before seasoning. The dollop of whipped cream in each bowl is a great success: it looks lovely and is full of flavor.

It may take some telephoning to find 8- to-10-ounce rainbow trout, but the effort allows you to serve whole fish just right for a single portion. Check prices. Recently these varied from $2.79 a pound (at Magruder's) to $4.49 a pound (at Chevy Chase Seafood). Trout come cleaned with their heads and tails and make an impressive presentation. You can snip off the fins with a pair of kitchen shears.

There is always some brief last-minute fussing with fish, since it must be cooked just before serving. Check to see whether the fish fit in the baking dish before buttering the pan. Often they take up more space than seems possible and I occasionally have had to transfer the butter and seasonings to a larger pan. Have the pan with the fish and seasonings prepared in advance. The fish go into the oven before you sit down for the soup -- exactly when depends on how cold the fish is, as colder fish take longer to cook. Time this maneuver so that the fish are cooked when the first-course plates are cleared. Then take several minutes in the kitchen to place the fish on a serving platter and make the sauce, quickly reducing the pan juices with wine, cream and lemon juice.

The tiny new potatoes, carved from larger potatoes, need nothing more than a sprinkling of parsley for color, and are useful for sopping up the delicious sauce.

The beet, red onion and tomato salad is an excellent accompaniment to the fish, both for the way it looks and the way it tastes. Three people who tried this salad at our house were sure they hated beets, but became converted. Instead of the mushy, watered-down objects they had learned to reject, they found a firm, full-flavored new vegetable.

The secret lies in baking the beets like potatoes. Two hours of baking in a toaster (or conventional) oven and the beets are done to perfection. Once they cool, it's nothing to slip off their skins and slice them. I prefer beets in the bunch rather than in bulk since freshness can be gauged by the state of the green tops. Cut off the stalks well above the beet -- it is critical not to cut into the skin of the beets because flavor and color will only bleed out -- and do not trim the tails until after they are baked. The greens, if in good condition, are delicious cooked according to any simple spinach recipe. Red onions are plentiful now and mild--perfect both with the beets and the tomatoes. Fresh dill is ideal with this salad.

Fruit is always a splendid ending to a meal. The many-textured dish combines both fresh and stewed peaches and pears with fresh berries, cherries and melon. The syrup in which the fruit is poached is used along with kirsch. No matter how much of this fruit is served, it somehow gets eaten. There is something joyful in a seemingly guiltless dessert. ICED (OR HOT) WATERCRESS SOUP (8 servings) 2 bunches watercress 6 tablespoons butter 6 tablespoons flour 6 10 3/4-ounce cans chicken broth 2 egg yolks Salt and pepper to taste 1/2 cup heavy cream Paprika (optional)

Wash the watercress, discard the coarse stalks and pat or spin dry. Melt the butter in a large saucepan, add the flour and cook together, stirring constantly, for 1 minute. Add the watercress to the pot and, still stirring, cook until the watercress is wilted. Turn the mixture into a food processor fitted with the steel blade and pure'e until very fine. Stop the motor several times and push the mixture down the sides of the bowl. Return the pure'e to the saucepan and whisk in the chicken broth, 1 can at a time. Bring the soup to a boil over moderate heat, stirring constantly. Turn off the heat. Beat the egg yolks in a deep bowl. Add 2 large ladles full of soup, pouring it in a thin stream and whisking constantly. Then in a thin stream, whisk the egg mixture into the saucepan. Stir the soup vigorously for 1 minute. The heat of the soup will cook the eggs. Season carefully with salt (the chicken broth is quite salty) and pepper to taste and set aside. When the soup cools to room temperature, refrigerate it for at least another 4 hours or overnight. If the soup is to be served hot, reheat it but do not let it come to a boil. Just before serving, whip the cream. Ladle the soup into bowls, top with a healthy dollop of whipped cream and, if desired, a sprinkling of paprika for color. RAINBOW TROUT WITH WINE AND CREAM SAUCE (8 servings) 8 small rainbow trout, about 8 to 10 ounces each, cleaned but with heads and tails left on 4 tablespoons softened butter 1/4 cup minced shallots or the white part of scallions 1/4 teaspoon thyme 2 bay leaves Salt and pepper to taste 1/4 pound (1 stick) melted butter 1/2 cup dry white wine 1 cup whipping cream 1 to 2 teaspoons lemon juice, or more if needed

Wash the trout, snip off the fins with kitchen shears and pat the fish dry with paper towels. Smear the softened butter over the bottom and sides of a roasting pan or gratin dish large enough to hold the trout in 1 layer. Measure the pan before buttering to make sure there is room for the fish. Sprinkle the shallots or scallions, thyme and bay leaves over the butter.. Place the trout in the pan and season them with salt and pepper to taste. Dribble the melted butter over the fish. If desired, the fish can be prepared up to this point, covered with plastic wrap and refrigerated for a few hours before cooking.

To cook the fish, place the pan in a 375-degree oven. Bake for 20 minutes if the fish are at room temperature or for 30 minutes if they are cold from the refrigerator. Baste the fish with the butter in the pan at least once during cooking. When the fish are done, remove them to a serving dish. Turn off the oven and place the serving dish in it, leaving the door open. Make the sauce by adding the white wine and cream to the pan and bringing to a boil. Cook, stirring, over moderately high heat, until the wine and cream have reduced and the sauce has thickened somewhat. Add 1 teaspoon lemon juice, taste and add more if needed. Discard the bay leaves. Remove the serving dish from the oven (use hot pads because the plate will be hot), spoon some of the sauce over the fish and turn the rest of the sauce into a bowl. Pass the remaining sauce separately. TINY NEW POTATOES (8 servings) 12 medium new potatoes Salt for the water 1 tablespoon minced parsley

Peel the potatoes and cut them into thirds, in chunks. Round off the chunks to make tiny potatoes and drop into a bowl of cold water. When ready to cook, bring a pot of water to a boil, add some salt and the potatoes and cook for 10 minutes, or until the potatoes are done. Drain and turn into a serving bowl. Sprinkle with parsley. BEET, RED ONION AND TOMATO SALAD (8 servings) 4 medium beets 1 large red onion, peeled 2 large tomatoes, blanched in boiling water for 20 seconds, cored and skinned 4 tablespoons olive oil 1 tablespoon lemon juice 1 clove garlic, crushed Salt and pepper to taste 2 tablespoons minced fresh dill or parsley

Trim the stalks above the beets, being careful not to cut the skin of the beets. Do not trim the tails. Reserve the greens for another use. Place the beets in a small pan and bake them in a 400-degree oven for 2 hours, or until a long, thin knife pierces the beets easily. Remove and allow to cool at room temperature. Zip the skins off the beets and trim their tops and bottoms. Slice the beets thinly and arrange them down one side of a long serving platter.

Slice the red onion thinly and arrange down the middle of the platter.

Slice the tomatoes thinly and place down the length of the other side of the platter.

Combine the oil, lemon juice, garlic, salt and pepper and drizzle over all. Sprinkle with dill or parsley. FRESH AND STEWED FRUIT IN SYRUP AND KIRSCH (8 servings) A bowl of cold water with 1 tablespoon white vinegar 6 large peaches 3 pears 3/4 cup sugar 1 cup water 1/2 teaspoon vanilla extract 1 cantaloupe 1/2 pound sweet cherries, pitted 1 pint blueberries, washed, picked over and drained well 1 pint strawberries, washed, hulled and halved 1/2 cup kirsch, or more, to taste Fresh mint sprigs (optional)

Blanch the peaches in boiling water for 20 seconds, skin, pit and halve them and place them in the bowl of water with the vinegar. Peel the pears, halve and core them and add to the bowl with the peaches.

Combine the sugar, water and vanilla extract in a saucepan, bring to a boil and cook until the syrup is clear. Remove 8 of the peach halves from the bowl, rinse under cold water, drain and add to the syrup. Poach the peaches for 4 minutes, turning them over after 2 minutes. Remove to a clean, large bowl with a slotted spoon. Rinse 4 of the pear halves under cold running water, drain and add to the syrup. Poach for 3 minutes, turning them after a minute and a half. Remove with slotted spoon and combine with poached peaches. Pour the syrup over the stewed fruit and allow to cool.

Cut the stewed peaches and pears into chunks. Return to the syrup. Rinse the 4 remaining uncooked peach halves and the 2 pear halves under cold, running water, drain, cut into chunks and add to the stewed fruit.

Seed the cantaloupe, cut the flesh into small balls with a melon baller and add to the bowl. Add the cherries, blueberries and halved strawberries along with the kirsch. Stir gently and taste to see whether more kirsch is needed. Refrigerate for up to 4 hours. One hour before serving, remove from refrigerator and let stand at room temperature. Turn into serving bowl and garnish, if desired, with fresh mint sprigs.