Cut the asparagus in the lower third. Bring plenty of water with the halved lemon, sugar, salt and butter to the boil. Cook the asparagus in it for about 4 - 5 minutes. Carefully place the asparagus in cold water and allow to cool. Then drain on a cloth.
Chop the basil very finely and mix with the parmesan, créme frâiche and egg yolk, season with salt and pepper.
Spread the well-drained asparagus on a baking dish with butter. Pour the parmesan cream on top and bake in the oven, covered to 180 ° C, for about 5 minutes, so that the paste can melt well. Then bake with the top heat or with the grill until the cream is brown and crispy.