Wash asparagus and cut off the tips. Put aside, these are added to the soup later. Roughly cut the rest of the asparagus into pieces.
Put asparagus in a saucepan with water and cook for about 5 minutes. (Use hot water from a water heater to speed it up)
Meanwhile, melt the butter in a pan. Add the flour and stir with a whisk on medium heat for about 1-2 minutes until it is a smooth mixture. Add cold milk and stir until a thick sauce is formed. Remove from stove.
Purée asparagus and water with a blender. Pour the roux into the soup and stir well with the whisk (or blender). Season with herbal salt as desired.