To make crust: Heat oven to 425° F. In food processor, whirl together flour and salt. Add butter to processor, pulsing just until mixture resembles coarse meal. Add egg yolk and water, 1 teaspoon at a time; process just until dough clumps together and begins to form a ball.
Set 2 pieces (each about 12-by-15 inches) plastic wrap on work surface. Lightly dust each with flour. Gather dough and press into a flat disk about 6 inches wide. Set disk in center of 1 piece of wrap. Set remaining plastic wrap, floured-side down on dough. With rolling pin, roll dough evenly into about an 11- to 12-inch round; occasionally turn round over, lift wrinkled plastic wrap, pull smooth and replace; continue rolling as needed.
To fill tart: Peel off top sheet of wrap and invert onto 2 X 15-inch baking sheet lined with cooking parchment. Remove remaining plastic wrap, Distribute berries in center of pastry, leaving about a 2-inch border. In small bowl, mix sugar and cornstarch. Sprinkle 3 tablespoons mixture evenly over fruit. Gently fold edges of pastry over berries, pleating edges to fit; leave an opening of 4 to 6 inches wide in center. Brush pastry with reserved egg white from crust. Sprinkle pastry lightly with remaining sugar mixture.
Bake tart on the bottom rack of the 425° F oven until crust is golden on bottom and juices bubble, 25 to 30 minutes. Cool on pan at least 15 minutes. While still warm, with wide spatula loosen tart from pan especially where juices have leaked out. Transfer to serving platter and serve warm or cool. Cut in wedges. Dust with powdered sugar, if desired.