Heat oven to 425° F. Oil a 9-inch springform pan or pie plate; set aside.
In a medium saucepan, bring 2¼ cups water to a boil.
Whisking constantly, slowly add the polenta. Reduce heat to low and cook, stirring, until the polenta starts to pull away from the side of the pan, 4 to 5 minutes.
Stir in the Parmesan, ½ teaspoon salt, and ¼ teaspoon pepper. Using a spoon, spread the polenta over the bottom and up the sides of the prepared pan.
In a bowl, combine the radicchio, tomatoes, and mozzarella, the remaining tablespoon of oil, and ¼ teaspoon each salt and pepper.
Spoon the mixture over the polenta and bake until the polenta is crisp around the edges, 20 to 25 minutes.