Wash and peel the asparagus and cut off the woody ends. Dice butter and melt in a pot. Wash lemon hot, grate dry and grate peel. Halve lemon and squeeze 1 half.
Remove butter from the stove and let it cool down a little. Boil up about 2 litres of water together with salt and 1 teaspoon of sugar in a larger pot. Add the asparagus and cook at low heat for about 15 minutes.
Prepare the pasta in boiling salted water according to the instructions on the packet. Wash the fish and pat dry. Heat oil in a coated pan. Fry the fish in it from both sides for 6-8 minutes. Season with salt and pepper.
Whisk egg yolks, 2 tablespoons lemon juice and white wine in a hot water bath until frothy. Gradually fold in the liquid butter. Stir in mascarpone and season to taste with 1 pinch of sugar, salt and pepper.
Drain the pasta and let it drain. Arrange asparagus, noodles and sauce on preheated plates and garnish with chervil. Cut remaining 1/2 lemon into slices and add.