Peel, wash and chop the potatoes. Cook in boiling salted water for about 20 minutes until soft
Clean the mushrooms, wash and halve them. Clean, wash and cut spring onions into rings. Heat 2 tablespoons of oil in a large frying pan. Fry 1/3 of the mushrooms for about 5 minutes while turning. After approx. 3 minutes add 1/3 of the bacon and fry, remove. Fry the remaining mushrooms with the remaining oil and bacon in the same way. Finally add spring onions to the last portion, stir in sour cream and 4 tablespoons of milk. Add fried mushrooms again. Let simmer for about 1 minute. Season to taste with salt and pepper. Spread the mushroom cream in an ovenproof dish
Drain the potatoes. Add fat and 200 ml milk and mash with a potato masher until mashed. Season to taste with salt and nutmeg. Spread the puree on the vegetables and spread it loosely. Sprinkle with cheese. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 40 minutes. Take out and serve immediately