Wash the potatoes thoroughly and peel them if necessary. Cook in boiling salted water for 15-20 minutes. Peel the cucumber, halve lengthwise, remove seeds and cut into slices. Peel and finely chop the onion. Clean, wash and halve or quarter the mushrooms.
Cut meat into thin strips. Fry in hot oil for about 5 minutes. Remove from the pan. Sauté onion and mushrooms in frying fat. Add cucumber slices and steam briefly. Pour on stock, add crème fraîche and mustard and cook for about 8 minutes. Bring to the boil and stir in the sauce thickener. Season with salt and pepper, add meat and warm up. Drain the potatoes and let them drain. Fry in hot fat until golden brown.
Pour on stock, add crème fraîche and mustard and cook for about 8 minutes. Bring to the boil and stir in the sauce thickener. Season with salt and pepper, add meat and warm up. Drain the potatoes and let them drain. Fry in hot fat until golden brown. Wash parsley and chop coarsely except for a few leaves for garnishing. Arrange the slices with potatoes on plates, sprinkle with chopped parsley and garnish with leaves