Wash the lemon hot, dry and finely grate the peel. Squeeze the lemon. Melt the butter in a small pot, put aside. Wash the chervil, shake dry and pluck off the leaves.
In a tall mixing bowl, puree the egg yolk, crème fraîche and lemon juice with a hand blender. Add butter in a thin stream while stirring to create a thick sauce. Stir in lemon zest, season with salt and pepper. Sprinkle with chervil. Serve with: new potatoes, pastrami, raw or cooked ham.
As a main course, calculate 500 g asparagus per person. For an appetizer, about 200 g asparagus is sufficient - but then cut the ingredients for the sauce in half.
Wash the asparagus, cut off the woody ends. Boil up salted water with 1 teaspoon sugar. Cook the green asparagus for about 8 minutes.