1. Rinse tomatoes, cut in half lengthwise, and place cut side up in an oiled 10- by 15-inch pan. Sprinkle lightly with pepper. Spread about 1/2 teaspoon pesto onto cut side of each tomato half.
2. Bake tomatoes in a 350° regular or convection oven until browned on top and slightly shriveled, 1 1/2 to 2 hours (if pan juices begin to scorch, add a few tablespoons water to pan). Let tomatoes cool about 10 minutes.
3. Place polenta rounds slightly apart on an oiled 12- by 17-inch baking sheet. Sprinkle 1/2 cup parmesan cheese evenly over rounds. Set a tomato half, pesto side up, on each and sprinkle remaining 1/2 cup cheese on top.
4. Bake polenta pizzas in a 450° regular or convection oven until cheese is melted and beginning to brown, 10 to 13 minutes. Let cool 2 to 3 minutes, then transfer with a spatula to a platter.