In a large skillet, melt butter over medium-high heat. Add mushrooms; sprinkle with salt and pepper. Cook, stirring frequently, until mushrooms begin to soften and release their liquid, about 6 minutes. Remove from skillet. Wipe skillet clean.
Add crème fraîche and cream to skillet, whisking until smooth. Bring to a simmer over medium heat; cook until slightly thickened, about 3 minutes. Remove from heat. Whisk in chives, thyme, and mustard; gently stir in mushrooms. Serve immediately.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/roasted-cornish-hens-with-mushroom-creme-fraiche-sauce-recipe/