Combine half the onions with the lime juice in a non-metallic bowl and set aside to pickle.

Heat the oil in a non-stick pan over a medium heat and cook the remaining onions for a few minutes until softened. Add the garlic and spices, stir for a few seconds, then tip in the beans along with the liquid from the can. Stir in the chopped tomatoes, tomato purée and bouillon powder, and cook over a low heat for 8 mins, or until the beans have softened and the liquid has reduced. Mash the beans roughly to break some of them down and thicken the mixture.

Rub a drop of oil over a large non-stick frying pan using a sheet of kitchen paper, and put one tortilla in the pan. Spread over the bean mixture, then cook over a low heat for 2 mins until the tortilla is crisp and golden. Stir the avocado, coriander and pickled onions together, then scatter this over the bean mixture and press the other tortilla on top, tucking the edges in to enclose the filling. Carefully flip over using a wide spatula and cook for 2 mins more until crisp. Slide onto a plate and cut into triangles. Serve with the lime wedges for squeezing over, and garnish with coriander sprigs, if you like.