Melt butter in a large skillet over medium high heat.
Add onions and saute for about 10 minutes until translucent.
Add mushrooms, season with salt and pepper and cook for another 15 minutes or until browned and tender.
Add garlic and stir for a minute, until fragrant.
Pour in the wine (or broth) and deglaze the pan, scraping up any browned bits from the bottom of the pan. Bring heat to low and continue to reduce wine slowly over low heat for approximately 2 minutes.
Add cream and gently simmer until sauce is slightly thickened. Season with salt and pepper.
Serve warm. Great over chicken, steak, pasta or mashed potatoes.