Clean and finely chop the mushrooms and onion.
Melt butter in a pan and sauté the onions first.
Add the mushrooms and stir-fry until soft.
Add flour and mix well, then cook for a bit longer.
Pour in milk and water (or chicken stock) and bring to a boil.
Season with salt and pepper to taste.
Blend until smooth.
Bring it back to a boil to finish.