Wash and peel the asparagus. Cut off the woody ends. Put asparagus in plenty of boiling salted water. Add sugar and 5 g fat and cook the asparagus for 15 - 20 minutes. In the meantime, clean and slice the mushrooms.
Peel and finely chop the onion. Heat 40 g of fat in a pot and fry the onion in it until it becomes glassy. Add the mushrooms, brown lightly and season with salt and pepper. Dust with flour and sauté briefly.
Add stock and milk while stirring, bring to the boil and simmer for about 5 minutes. Mix egg yolk and cream. Add 50 g Parmesan cheese to the hot sauce, dissolve in it and alloy the sauce with egg yolk. Do not let the sauce boil anymore and season with salt, pepper and nutmeg.
Lift the asparagus out of the water and drain well. Roast the slices of toast and place them on a baking tray lined with baking paper. Thickly cover roasted toasts with turkey breast and asparagus and spread the sauce on top.
Sprinkle with 25 g cheese and spread 15 g fat in flakes on top. Bake in the preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: level 4) for approx. 15 minutes. Serve the asparagus toasts immediately, garnished as desired with salad and chervil.