Boil the beans for 15 minutes, then rinse and drain and put them in a bowl. Pour fresh boiling water over them and leave for one hour. Rinse and drain them again.
In an enamelled casserole or other pot suitable for both stove top and oven, gently fry the onion in the oil until beginning to brown. Add the spices and cook for 2 to 3 minutes. Stir in the tomato purée, and cook until the mi