Wash the potatoes and cook covered for about 20 minutes. Drain, rinse with cold water and let cool down briefly. Peel and let potatoes cool down.
Put the prawns in a sieve, rinse and drain. Roughly grate the potatoes on a grater. Mix with prawns, egg and flour. Heat 1 tablespoon butter in a large coated pan (28 cm Ø).
Spread the potato mixture in it. Squeeze it together with the spatula to form the flat cake. Season with salt and pepper.
Fry over medium heat for about 10 minutes. Turn and fry on the other side for another 10 minutes.
Clean and wash the mushrooms, and if necessary wash them briefly. Cut large mushrooms smaller. Clean, wash and cut spring onions into rings. Heat 1 tablespoon of oil in another pan. Brown the ham cubes in it.
Take it out. Heat 1 tablespoon of oil in the frying fat and fry the mushrooms in it for about 5 minutes in two portions while turning. Add spring onions and fry for 2-3 minutes. Add ham again and stir in crème fraîche.
Let it thicken a little. Season with salt and pepper.
Divide Rösti in the pan into 4 pieces and serve with mushroom sauce.