Meanwhile, in a large, deep skillet, heat the 3 tablespoons of olive oil. Add the onion and bell pepper and cook over moderately high heat, stirring, until softened, about 5 minutes. Add the ground beef and cumin and cook over high heat, stirring occasionally, until the meat is browned and any liquid has evaporated, about 8 minutes. Add the flour and cook until no floury taste remains. Stir in the tomatoes and stock and simmer until thickened, scraping up any browned bits from the bottom of the pan, about 3 minutes.