In food processor, pulse flour, butter, cornmeal, 1/4 cup Parmesan, salt and peppers until crumbly. With motor running, add buttermilk in a slow, steady stream; process just until dough forms a ball. Divide dough in half, shape each half into a disc, wrap in waxed paper, and chill 1 hour or overnight.
Preheat oven to 400°. Set out a 13 1/2" x 4 1/2" rectangular or a 9-inch round tart pan. Roll out one disc of dough to a 1/4-inch-thick 15" x 6" rectangle or 11-inch circle; pat dough into pan. Brush with egg white. Add basil to chilled tomato mixture and spread it evenly over tart shell. In a small bowl, stir together egg yolk and 2 teaspoons water. Roll out remaining half of dough to 1/4 inch thick and cut into 1-inch strips; arrange in crisscross pattern over top of tart, brush with egg yolk mixture, and sprinkle with 2 tablespoons Parmesan.
Bake on a baking sheet 25 to 30 minutes, or until crust is browned. Serve warm or at room temperature.
Adapted from Back to the Table by Art Smith