Arrange a layer of tomato slices in bottom of a lightly greased 2-quart casserole. Combine onion and green pepper. Spoon half of mixture over tomato slices. Combine sugar, salt, and pepper; sprinkle half of mixture over vegetables. Place a layer of breadcrumbs over mixture. Repeat layers, ending with breadcrumbs on top. Dot top of casserole with butter.
Bake at 375° for 25 minutes. Sprinkle cheese over top; continue baking an additional 5 minutes.