Bring a large pan of water to the boil. Add the rice and simmer for 25 minutes until tender.
Meanwhile, put half the tomatoes in a separate pan with 150ml water. Bring to the boil and simmer gently for 15 minutes until the tomatoes start to break down.
Meanwhile, heat the oil in another pan. Add the onion and garlic and cook for 5 minutes until soft. Stir in the chilli powder, then remove from the heat and stir in the black beans and mixed beans.
Drain the rice and transfer to a serving platter. Spoon over the cooked tomatoes and beans, then top with the remaining tomatoes, avocado and spring onions. Season with freshly ground black pepper and serve scattered with the chopped coriander leaves.