Mix the tomatoes, ¼ onion, lime juice and 1 tbsp oil and set aside. Fry the remaining onion in 2 tbsp oil until it starts to soften. Add the garlic, fry for 1 min, then add the cumin and chipotle and stir until fragrant. Tip in the beans and a splash of water, stir and cook gently until heated through. Stir in most of the tomato mixture and cook for 1 min, season well and add most of the coriander.
Toast the bread and drizzle with the remaining 1 tbsp oil. Put a slice on each plate and pile some beans on top. Arrange some slices of avocado on top, then sprinkle with the remaining tomato mixture and coriander leaves to serve.