Clean fish fillet, acidify, season with salt and season with pepper. Place fish pieces in a saucepan, without adding fat and liquid, let them bubble up gently on 2 or possibly automatic cooking plate 4-5, the fish draws water, then turn off the hotplate and steam in the closed saucepan in about 15 min until soft.
Cool down the fish. Mix mayo with sour whipped cream, tomato ketchup, . Salt and pepper. Add grated apple (briefly pass through cold salted water to prevent discoloration) and pineapple pieces to the mayo.
Spread out a plate with lettuce leaves. Cut the fish into pieces and arrange on top, alternating with the apple mayonnaise.