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 16 small Rösti over medium heat until golden brown.
Remove the finished Rösti and let them drip off on kitchen paper. Wash and drain the crabs. Clean and wash spring onions and cut into fine rings. Clean and wash the celery, put some leaves aside for garnishing.
Cut the rest of the celery into thin slices. Halve avocado and remove core. Spoon the flesh from the skin. Cut the flesh into pieces and sprinkle with lemon juice immediately. Put 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 Rösti and salad on plates, garnish with celery leaves.