Soak the black beans in cold water overnight, rinse under cold water and drain. Place them in a pan, cover with water and bring to the boil. Reduce the heat, simmer for 1½ hours or until tender. Drain and leave to cool thoroughly.
Chop the beans coarsely, then place in a bowl with the onion, tomatoes, sweetcorn, mango and coriander and toss well to combine.
For the dressing, pla