To remove excess salt from the fish, bring the fish to a boil in a large pot of water. Drain the water, and repeat the process. Drain the fish, flake and press liquid out.
In a large frypan, saute oil, onions, tomatoes and capsicums for 2 minutes. Mix in flaked fish, lemon juice and chilli flakes. Cook mixture at a low heat until the moisture has almost completely evaporated.
Pile the mixture into the center of a serving dish, and arrange lettuce and avocado around it.