Prepare the potatoes with mushrooms - heat the oil in a frying pan over medium heat for about 2 minutes. Place the onion in the pan. Stir-fry until lightly golden, about 5 minutes. Add the mushrooms to the onion and stir well. Fry over medium heat for about 5 minutes.
Combine the mushrooms and potatoes. Transfer the potatoes to the pan with the mushrooms and onions. Mix the vegetables well and fry, stirring, for 15 minutes. Add salt, black pepper and stir. After 3 minutes, turn off the stove.