Melt butter in large stock pot over low heat until foaming. Add onion and sauté until translucent. Add mushrooms and cook for five minutes on medium low heat, stirring frequently. Season with salt and pepper. Add broth and bring to a boil. Reduce heat and simmer for 45 minutes. Add rice and heat through. Stir in half and half and nutmeg. Sprinkle with parsley before serving.