In a large nonreactive skillet, melt 5 tablespoons of the butter. Add the mushrooms and cook over moderately high heat until lightly browned. Stir in the shallots, garlic, celery, onion and 1 1/2 teaspoons of the thyme. Cook until all the vegetables are softened slightly. Stir in the wine and simmer until almost evaporated. Add 2 cups of the Chicken Stock and simmer over moderate heat until the liquid reduces and thickens, about 13 minutes. Season with salt and pepper and transfer the mixture to a large bowl.