Put the onion in a bowl and cover with half the lime juice and a good pinch each of sugar and salt. Leave to soften while you get everything else ready.
Mix together the crabmeat, spring onions and half the chilli. Season with black pepper and set aside. Mash the avocado with the remaining lime juice, garlic and some seasoning. You can leave it quite chunky or mash it until smooth. Stir in the rest of the chilli.
Bend each tortilla in half and toast in a toaster for 1 min. Put on 2 plates and top with the salad leaves, then mashed avocado. Finish with the crabmeat and drained pickled onion. Serve with lime wedges for squeezing over.