Cut the fish into strips the length of your finger but half as thick. Cover with lime juice. Turn over and marinate for 2 hours. Drain and sprinkle with the fennel and salt.
Cut the avocado into quarters, remove the skin and thinly slice lengthways. Sprinkle the avocado with some lime juice and a little sea salt and arrange on four plates.
Mix all the salsa ingredients just before you serve the dish then put a pile on each plate with the fish. Serve immediately.
