Sauté kidneys and let them drain. Put chunk of butter into skillet. Add 2 minced shallots (or 1 onion), 2 cups sliced mushrooms, and 1 cup white wine. Boil, reducing liquid to half. Season with salt and pepper. Add liaison of 2 egg yolks and 3 tablespoons crème fraîche or whipping cream. Add sautéed kidneys. Stir-cook until hot, but do not boil. Taste and adjust seasonings. Serve on toast or in croustades as first course.