Heat the olive oil in a large skillet. Add the onion and cook over low heat until softened, about 4 minutes. Add the garlic, green bell peppers, jalapeños and cayenne pepper and cook, stirring occasionally, until the vegetables are softened, about 20 minutes. Add the tomatoes and cook over moderately high heat, stirring frequently, until the juices have evaporated, about 7 minutes. Season with salt and black pepper and stir in the cilantro.