Cook the asparagus 15 minutes. Save the cooking liquid. Cut the end 5 centimeters off and chop these. Brush the ham with mustard and make rolls with a slice of ham around each three asparaguses. Put these rolls in an oven dish. Stir fry the onions in the butter, add the flour and simmer for 3 minutes. Add 1 cup of cooking liquid and the cream, bring to the boil and simmer for 3 minutes. Add the cut asparagus ends and salt to taste. Pour this sauce on the asparagus rolls and bake 15 minutes in the oven at 180 degrees Celsius.