Wash and peel the asparagus, then cut into 3cm pieces. Boil water with a pinch of salt and sugar and cook asparagus 20-25 minutes until very soft.
Drain the asparagus, retaining the cooking water. Puree the asparagus with 1/2 of the water.
Melt the butter in a saucepan and stir in flour. Fry lightly, then pour in the milk and beat with a whisk until it thickens. Add the pureed asparagus and remaining cooking water to desired thickness. Boil again, then add the cream, salt and pepper to taste.