Heat a large heavy skillet over high heat. Add the butter, garlic and shallots and cook for 30 seconds. Add in the mushrooms and sauté until browned over medium high heat, about 6-8 minutes.
Add in the flour and stir for 30 seconds until well incorporated. Add sherry wine and chicken stock to deglaze pan, be sure to scrape up all the browned bits on the bottom. Add cream and cook on medium high heat, about 5-10 minutes, until reduced by 1⁄2 or desired sauce consistency.
Season mushroom sauce with salt & pepper to taste.