For the Rösti, peel, wash and coarsely grate the potatoes. Mix with egg yolk. Season with salt, pepper and nutmeg. Wash and peel the asparagus and cut off the woody ends briefly. Cook the asparagus in boiling salted water with some sugar and lemon juice at medium heat for 15-20 minutes. In the meantime, form about 12 Rösti from the potato shavings and place them on a baking tray lined with baking paper.
Bake in a preheated oven (electric: 200 °C/ gas: level 3) for 12-15 minutes. Turn once. Wash, clean and slice the mushrooms. Wash the slices, dab dry and cut into thin strips. Peel and finely chop the onion. Heat the fat. Fry the onion, sausage and mushrooms in it in portions until golden brown. Season with pepper. Deglaze everything with 400 ml asparagus stock and cream and heat up. Stir flour and 3 tablespoons of water until smooth. Add to the sauce while stirring, bring to the boil again and season to taste.
Heat the fat. Fry the onion, sausage and mushrooms in it in portions until golden brown. Season with pepper. Deglaze everything with 400 ml asparagus stock and cream and heat up. Stir flour and 3 tablespoons of water until smooth. Add to the sauce while stirring, bring to the boil again and season to taste. Wash the chervil, dab dry and remove the leaves. Serve asparagus, shredded meat and rösti sprinkled with chervil