Peel the asparagus, bring the water with salt, sugar and lemon juice to the boil and cook the asparagus in it for about 10 minutes.
Chop the onion and sweat in the butter until translucent. Add flour, sweat. Pour in the broth slowly while stirring. Add the creme fraiche, stir in the chopped chervil. Season to taste with salt and pepper and possibly a little lemon juice.
Boil the potatoes as jacket or boiled potatoes and serve with them.