For this recipe, the grandest of all French pastries is formed into a free-standing rectanglar loaf slit on top and filled with berries. It is the most elegant way to dress a berry and should be served warm from the oven to insure that the pastry remains tender and flaky. It is called a jalousie, a word best left untranslated -- for "peek-a-boo" tart is belittling and calling it a "berry venetian blind" tart seems worse.

Now that good quality commercial puff pastry made with butter is available to the home cook, the difficult and time-consuming process of making the dough for this classic French dessert is no longer necessary. Frozen sheets of puff pastry are available in some of the area's better supermarkets and gourmet shops, and the dough is tender, buttery, and more flaky and high rising than most cooks could produce if they started from scratch. Best of all, for this elegant recipe you don't even have to roll out the dough. FRESH BERRY JALOUSIE (8 servings)

1 pound frozen puff pastry, defrosted overnight in refrigerator

Flour for work surface

2 pints raspberries, blackberries, or blueberries

1/4 cup confectioners'sugar

1 tablespoon cornstarch

1 egg, beaten well with 2 teaspoons of cold water for the wash

Cut puff pastry in half lengthwise and place one piece on a large baking sheet or jelly roll pan that has been lined with baker's parchment. Refrigerate while you prepare the second piece.

Place the second piece of dough on a lightly floured work surface and fold in half lengthwise. Cut dough with a sharp knife at 1/2-inch intervals along folded side, but do not cut all the way; leave 1 1/2-inch border. Set aside.

Dip a pastry brush into cold water and brush an inch-wide border around the piece of pastry on the baking sheet. Refrigerate.

Combine the berries, sugar and cornstarch and toss lightly together until berries are just dusted in powder. Pile the berries onto the center of the brushed dough, spreading the berries in a thick, even layer.

Using both hands, carefully lift the notched sheet of dough off the counter and place it over the berries. Stretch the dough as necessary to fit it in place over the berries and press the edges with your fingers just enough together. Refrigerate for at least 30 minutes, then crimp the edges of the jalousie all round by pressing firmly into the dough with the tines of a large fork. Trim the edges neatly with a sharp knife.

Brush top of dough all over with beaten egg wash. Bake in a 450-degree oven for 15 minutes, then reduce heat to 350 degrees and bake for about another 10 minutes until the jalousie is nicely browned all over.

Slip the jalousie onto a rack and cool for 5 to 10 minutes. Transfer to a large decorative serving plate and serve while still warm. Slice with a serrated knife.