Chop the garlic and onions finely, braise them in butter.
Clean and dice the mushrooms, add them to the garlic and onions, braise them until golden as well.
Add the flour and stir until it's absorbed by the fat and braised.
If needed, add more butter. Dilute with the milk while stirring energetically.
To achieve the desired density you may have to dilute with water/broth/milk.
You can also blend the soup if you like. Garnish as desired.