Put the soaked black beans into a large saucepan and cover with water. Bring to the boil, then simmer for 30 minutes until almost tender. (If using a tin of black beans, omit this step.) Drain and rinse the beans – cooked or tinned – then set aside in a bowl.
Meanwhile, heat the oil in a large frying pan over medium heat. Cook the onion and garlic for 5 minutes until the onion is soft.