Prepare all ingredients.
Peel carefully the asparagus.
Cut the base by several centimeters.
Drop the asparagus in salted boiling water and cook for about 20 minutes.
Drain and reserve.
Béchamel: Melt the butter in a saucepan, add the flour and mix thoroughly.
Cool this roux blanc and pour 40 cl of the asparagus water along with 25 cl milk.
Boil and cook 4 to 5 minutes stirring constantly. Add salt, pepper and a touch of nutmeg.
Turn on the grill of the oven. Place the asparagus in a buttered gratiné pan...
...then coat with the sauce.
Sprinkle with grated Swiss cheese. Bake under the grill of the oven and leave to gratiné.
Serve immediately.