Boil the potatoes in water for about 30 minutes. In the meantime clean and wash the broccoli and divide into small florets. Cook in boiling salted water for about 8 minutes. Clean, wash and dry mushrooms, chop oyster mushrooms.
Peel and finely chop the onion. Drain broccoli, rinse with cold water, drain. Heat the oil and fry the mushrooms in portions. Add onion, fry briefly. Put the whole mushrooms into the pan.
Add 250 ml water and cream, bring to the boil and stir in the stock. Stir in the sauce thickener, simmer for 1 minute. Add broccoli and season with salt and pepper. Drain potatoes, rinse with cold water and peel.
Cut a lid off the potatoes, hollow out the potatoes. Press the potato lid and the hollowed out potatoes through a potato ricer. Mix in egg yolk and 25 g cheese, season with salt and nutmeg.
Put about 2/3 of the mushroom ragout in an ovenproof dish, place the potatoes on top. Spread the rest of the ragout in the potatoes. Put the mashed potatoes into a piping bag with a star-shaped spout. Spray the mashed potatoes with the mashed potatoes.
Sprinkle with 25 g cheese. Bake in the preheated oven (electric cooker: 200 °C/ convection oven. 175 °C/ gas: level 3) for 10-15 minutes. Serve garnished with chervil.