Melt 2 tablespoons butter in a saucepan over medium heat
Add 1 small chopped onion and cook until soft
Add 2 cups sliced mushrooms and cook until browned
Stir in 2 tablespoons all-purpose flour
Cook for 1 minute
Slowly add 2 cups chicken or vegetable broth while stirring
Add 1 cup milk or cream
Bring to a gentle simmer
Cook until thickened
Season with salt and black pepper
Serve warm