Peel the potatoes and cut them into pieces. Cook in boiling salted water for about 20 minutes. Warm milk and butter. Drain the potatoes and let them evaporate briefly. Add milk mixture and mash everything to a puree.
Season to taste with salt and nutmeg, allow to cool slightly. Clean and trim the mushrooms, halve depending on size. Peel and finely chop the onion. Heat the oil in a large pan. Add the mushrooms and fry for 2-3 minutes, turning several times.
Add onion and bacon and continue to fry for 2-3 minutes. Add cream and simmer for 5 minutes. Season to taste with salt and pepper. Put approx. 7 tablespoons of puree into a piping bag with a large perforated spout.
Divide the rest of the purée into 7 greased ovenproof dishes (each containing approx. 150 ml). Add mushroom cream sauce. Spray 1 "Tip" of puree on each. Whisk the egg yolk and brush the "Zipfel" with it.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 150 °C/ gas: level 3) for about 15 minutes.