1. Drain the beans and pit them in a large saucepan. Pour in water to cover and place the lid on the pan. Bring to a boil, lower the heat slightly and simmer the beans for 45-50 minutes or until tender. They should still have a little bite and should not have begun to disintegrate. Drain, rinse under cold water, then drain again and transfer to bowl. Leave the beans until cold.
2. Soak the chipotle and pasilla chiles in hot water for about 10 minutes, until softened. Drain, reserving the soaking water. Remove the stems, then slit each chile and scrape out the seeds with a small sharp knife. Chop the flesh finely and mix into a smooth paste with a little of the soaking water.
3. Roast the garlic in a dry frying pan over medium heat for a few minutes, until the cloves start to turn golden. Crush them and add them to the beans.
4. Chop the onion and tomatillos and stir them into the beans. Add the chile paste and mix well. Add salt to taste, cover and chill before serving.
