Toast cumin, coriander and clove in a small skillet over a low heat for 2-3 minutes until fragrant. Crush into a fine powder in a mortar and pestle.
Heat oil in a large frypan and add onions. Cook 3 minutes until beginning to soften, then add the beans, spices and crushed tomatoes.
Bring to a simmer and cook 5 minutes, or until thickened and saucy. Season well with salt and serve with sour cream and extra cumin seeds sprinkled on top.