Heat broiler. On a rimmed broilerproof baking sheet, toss the tomatoes, onion, and oil; turn the tomatoes cut-side down. Add the peppers, cut-side down.
Broil until tender and charred, stirring the onions and turning the tomatoes and peppers halfway through, 5 to 8 minutes.
Heat oven to 400° F. In a food processor, puree the tomatoes, onion, cinnamon, ½ teaspoon salt, and ¼ teaspoon pepper until smooth. Spread half the sauce in a 9-by-13-inch baking dish. Arrange the peppers in the dish, cut-side up.
In a medium saucepan, bring 2 ¼ cups water to a boil. Add ½ teaspoon salt. Gradually whisk in the polenta. Cook, whisking constantly, until thickened, 3 to 4 minutes. Stir in the corn, cheese, and all but 2 tablespoons of the scallions.
Divide the polenta among peppers. Top with the remaining sauce and bake until heated through, 5 to 10 minutes. Sprinkle with the remaining scallions before serving.