Wash and peel the asparagus and cut off the woody ends. Cook the asparagus in approx. 1 l boiling, slightly salted water covered with some sugar for approx. 15 minutes
Clean the mushrooms, wash if necessary and cut into slices. Peel and finely chop the onion. Sauté in hot butter in a pot. Add the mushrooms and fry them well. Season with salt and pepper. Sprinkle with flour and sauté briefly. Stir in 1/4 l of the asparagus stock, milk and stock. Bring to the boil and simmer for about 5 minutes
Grate cheese. Melt 2/3 in the hot sauce. Mix egg yolk and cream. Pour the sauce over it (don't boil any more!). Season to taste with salt, pepper and nutmeg
Drain the asparagus well. Roast the toast and place it on a baking tray lined with baking paper. Cover with cold cuts and asparagus. Spread half of the sauce on top and sprinkle with the rest of the cheese. Bake the toasts in a preheated oven (electric oven: 225 °C/circulating air: 200 °C/gas: level 4) for 10-12 minutes. Serve with salad and chervil. Heat the rest of the sauce briefly and add