Add butter to a large skillet over medium high heat.
Add onions and sautee for about 15 minutes until translucent and slightly browned.
Add mushrooms, season with salt and pepper and cook for another 15 minutes or until browned and tender.
Add garlic and basil and give a quick stir.
Add wine and deglaze the pan. Bring heat to low and continue to reduce wine slowly over low heat for best flavor.
Add cream and continue to stir for about a minute. Plate creamed mushrooms and onions and garnish with parsley.