Mix mayonnaise, lemon juice and mustard. Stir in yoghurt, season with salt and pepper. Put it in a cold place. Wash, dry and halve the lemon. Squeeze one half, cut the second half into thin slices.
Rinse the crabs with cold water, drain them and sprinkle them with lemon juice. Marinate for 10 minutes. Wash the lettuce leaves and dab dry. Wash the dill and chop finely, except for a little for garnishing.
Toast roasting. Mix dill and fat and spread on the bread. Cover the toast slices with the salad leaves. Drain the crab meat and spread on the salad leaves. Put yoghurt mayonnaise on the crabs and cover with caviar.
Clean, wash and slice the tomato. Garnish toast with lemon slices, dill and tomato slices.