Mix mayonnaise and yoghurt. Season to taste with salt, pepper and some lemon juice.
Heat 2 tablespoons of oil in a coated frying pan. Fry the Rösti in portions according to the instructions on the packet. Remove and keep warm.
Wash the lettuce and drain. Wash, quarter, seed and finely dice the tomatoes. Mix 1 tablespoon lemon juice, honey, salt and pepper. Fold in 2 tablespoons of oil. Cut avocado in half, stone. Remove flesh, dice finely.
Mix salad, tomatoes, avocado and vinaigrette. Arrange everything with the crabs.