Take one buttery pie pastry, add sliced tomato, saut�ed vegetables, and lots of cheese, and you have the makings of a pretty snazzy dinner. This one is a little more involved than the other pizzas here, but worth the effort; you might want to make it when you're doing a little family-style entertaining or preparing a special weekend meal. To keep the assembly as easy as possible, make the pastry the day before and refrigerate it. Let the chilled pastry sit at room temperature for about 10 minutes before rolling it.
To make the pastry, put the flour, sugar, and salt in the bowl of a food processor; pulse to mix. Add the butter and pulse 6 to 8 times, breaking it into small bits. Put the yolk in a 1-cup glass measure. Add just enough water to make 1/8 cup of liquid; blend with a fork. Pour the liquid over the flour mixture. Pulse the machine again repeatedly until the mixture resembles damp, clumpy crumbs.
Empty the dough onto your counter and pack it like a snowball. Knead it once or twice, then place it on a large sheet of plastic wrap. Flatten it into a disk about I inch thick. Wrap it and refrigerate for at least 45 minutes.
To prepare the filling, heat the oil in a large skillet. Add the onion, mushrooms, and zucchini and sauté over moderate heat, stirring often, about 8 minutes, until the onion is clear. Salt and pepper the mixture to taste. Stir in the basil and remove from the heat.
When you're ready to assemble the dish, heat the oven to 375°. On a sheet of lightly floured waxed paper, roll the dough into a 13 1/2-inch circle. Invert the dough over a large baking sheet and peel off the paper. Imagine a circle 9 inches in diameter in the center of the dough; sprinkle the cornmeal and G cup of the Parmesan cheese over the area. Layer that area with the sliced tomato, then the sautéed vegetables. Fold up the uncovered edge of the dough over the filling.
Bake the pizza for 20 minutes on the center rack. Remove it from the oven and top with the rest of the Parmesan and the Cheddar cheese. Bake until the crust is golden brown, about 15 minutes longer. Cool briefly, then slice the pizza and serve. Makes 6 to 8 servings.