Meanwhile, in a large, deep skillet, heat the olive oil until shimmering. Add the onion, season with salt and pepper and cook over moderately high heat until softened and just beginning to brown, about 5 minutes. Add the bell peppers, capers and crushed red pepper and cook, stirring occasionally, until the peppers are softened and lightly browned in spots, about 10 minutes. Stir in the vinegar and cook until nearly evaporated, about 2 minutes. Add the tomato sauce and cook for 5 minutes.