Peel the asparagus spears from the heads (a little more generously in the lower third), removing the woody ends. Cook gently in a mixture of water, butter, salt and sugar for 15-20 minutes, then drain. Catch the asparagus stock and keep the asparagus warm.
For the sauce, bring the asparagus broth, butter and the cornstarch dissolved in a little water to the boil, stirring constantly. Remove the saucepan from the heat and fold in the whipped cream with the egg yolk. Season to taste with lemon juice, salt and nutmeg, do not let it boil anymore!
Pour the cream sauce over the asparagus, sprinkle with a little parsley, or serve the sauce separately.
As an accompaniment to ham, steaks or fish. Increase the amount with potatoes as a main course. Half the amount of the recipe is sufficient as a starter.