Wash the potatoes and boil in water for 20 minutes. Then rinse with cold water, peel and let cool. In the meantime peel onions and cut them into rings. Clean the mushrooms, wash if necessary, cut into slices.
Heat two tablespoons of oil. Brown the onions and mushrooms in it. Season with salt and pepper and cook over medium heat for ten minutes. Add 150 to 200 millilitres of water or broth and cook for another five minutes.
In the meantime cut the potatoes into slices. Heat two tablespoons of oil. Fry the potatoes for twelve minutes while turning. Season with salt, pepper and sugar. Dab the fillet dry and cut into fine strips.
Heat a tablespoon of oil in a frying pan. Stir-fry half of the meat for two minutes over a high heat, then add to the vegetables. Prepare the second half of the meat in the same way. Season with salt, pepper and mustard.
Stir in sour cream by the spoonful at low temperature, season to taste again. Serve Boeuf Stroganoff and fried potatoes in portions, sprinkled with cucumber strips and dill.