Cut the halibut into medium sized pieces and season. Wash the lettuce leaves and spin to dry. Separate the mango flesh from the skin, core and cut into slices. Fry the fish in the butter or oil for 2-4 minutes, depending on the thickness of the fish. Carefully mix together the fish, lettuce and mango. Season with salt and pepper and moisten, if necessary, with a little vegetable stock. Cut the toasted bread into cubes and sprinkle over the salad.