Wash the potatoes and cook covered for about 20 minutes. Drain, peel and cool
Peel onions and cut them into rings. Clean, wash and slice the mushrooms. Wash the fillet and cut into fine strips. Cut potatoes into slices. Fry in 2 tablespoons of hot oil for about 15 minutes until crispy. Season with salt and pepper
Heat 2 tablespoons of oil in a frying pan. Fry the meat in portions at high heat for 2-3 minutes. Season with salt and pepper. Take out. Heat 1 tablespoon of oil in the frying fat. Fry onions and mushrooms for about 10 minutes, season. Stir in a good 200 ml water, stock and mustard. Bring to the boil and simmer for about 5 minutes
Stir in sour cream bit by bit (do not boil any more!), season to taste. Arrange everything. Garnish with cucumber and dill