Heat the oil in an ovenproof casserole and cook the onions until softened, about 5 minutes, stirring occasionally. Stir in the chili powder and cook for 1 minute. Stir in the tomatoes, bouquet garni, lemon juice, and salt and pepper. Bring to a boil. Reduce heat and simmer for 20 minutes, stirring occasionally. PurÄe in a blender or food processor.
