Melt the butter in a large saucepot over medium-high heat. Add the mushrooms and cook, stirring occasionally, until dry and tender, about 10 minutes; season with salt and pepper.
Sprinkle the flour over the mushrooms and cook, stirring, until lightly browned, about 1 minute. Add the broth and sherry and continue to cook until it comes to a boil and thickens, about 5 minutes. Stir in the heavy cream and thyme and adjust the seasoning with salt and pepper.
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