Polenta Pizzas with Roasted Tomatoes and Olives

Line 15x10-inch rimmed baking sheet with foil. Combine 4 cups water and 1 teaspoon salt in heavy large saucepan. Bring to boil. Gradually add polenta, whisking until smooth. Reduce heat to medium-low; cook until polenta thickens and pulls away from sides of pan, whisking frequently, about 30 minutes for polenta and 15 minutes for yellow cornmeal. Whisk in butter and 1 teaspoon oregano. Pour out polenta onto prepared baking sheet; spread evenly to edges of sheet. Chill until firm, at least 2 hours or overnight. Preheat oven to 300 °F. Line another large baking sheet with parchment paper. Arrange tomato slices in single layer on prepared sheet. Sprinkle tomatoes with 1/2 teaspoon sugar, then some salt. Drizzle with 2 tablespoons oil. Roast until tomatoes are tender but sides are firm and slices still hold together, about 2 hours. Cool. (Polenta and tomatoes can be made 1 day ahead. Cover separately and chill.) Preheat oven to 450 °F. Line another large baking sheet with parchment paper. Using 1 3/4- to 2-inch-diameter cookie cutter, cut out 24 rounds from polenta. Place rounds in single layer on prepared sheet. Top each round with 1 tomato slice. Sprinkle with remaining 1 teaspoon oregano. Top each with 2 or 3 olive halves. Drizzle with remaining 2 tablespoons oil, then sprinkle with Parmesan. Bake polenta pizzas until heated through and cheese melts, about 10 minutes. Serve warm.