Heat a little oil in a saucepan, and saute the onion until softened. Add the garlic and chopped fresh chili, if using, and cook another couple of minutes, then add the cumin powder (and chili powder, if using) and cook another minute or until aromatic. Then add the tomatoes and beans, bring to a simmer, and cover and cook for about 15 minutes, or until beans are quite soft. Using a potato masher, crush about two thirds of the beans into a paste in the saucepan. Keeping the lid off, cook the chili for another 10 minutes or so, stirring occasionally, and adding a trickle of water if it starts to catch. Taste for seasoning. Stir through chopped coriander before serving.

This is a good standby for nights when you get home late and there are no vegetables in the fridge. But I often plan to make it, and serve it with guacamole, salsa fresca, and tortillas - delicious.