Peel and wash the potatoes and grate them roughly in a bowl. Stir egg and flour into the potatoes and season with salt and nutmeg. Heat the oil in portions in a pan and fry about 24 small Rösti over medium heat until golden brown.
Remove the finished Rösti and let them drip off on kitchen paper. Clean and wash the salads, pluck into bite-sized pieces and drain. Wash and drain the crabs. Clean and wash spring onions and cut into fine rings.
Clean, wash and thinly slice the celery. Halve avocado, remove core. Spoon the flesh from the skin. Cut the flesh into pieces and sprinkle with lemon juice immediately.
Put salads, spring onions, crabs, celery and avocado in a bowl. Mix mayonnaise, yoghurt, milk, Worcester sauce and ketchup, season to taste with salt and pepper and carefully fold into the salad.
Arrange salad with Rösti on plates.