Make the crust: Pulse flour, cornmeal, sugar, and salt in a food processor. Add butter; process until mixture resembles coarse meal. With machine running, add ice water in a slow, steady stream; process just until dough holds together. Divide dough in half, wrap in plastic, and refrigerate 1 hour (or up to 3 days).
Preheat oven to 350 degrees. On a lightly floured surface, roll out dough to 1/8 inch thick. Cut out rounds to fin in your tart pans. Place on a baking sheet, and refrigerate 30 minutes.
Make the filling: Heat oil in a large skillet over medium heat. Add bell peppers, and cook until just tender, 4 to 5 minutes. Add zucchini, squash, eggplant, and onion and cherry tomatoes cook, stirring, 2 minutes. Add sherry and salt; season with pepper. Cook, stirring, until liquid has evaporated. Add oregano and garlic powder.
Spoonb vegetable mixture into each tart shell. Bake until crust is golden brown, 40 to 50 minutes.