In the same skillet, brown the mushrooms and onion in 60 ml (1/4 cup) of the butter. Season with salt and pepper. Sprinkle with the flour and cook for 1 minute, stirring constantly. Add the milk and broth and bring to a boil, stirring constantly. Add the peas, 250 ml (1 cup) of Parmesan cheese and the nutmeg. Adjust the seasoning.