1. Heat a large skillet over medium-high heat. Coat pan with cooking spray. Add the mushrooms and thyme to pan; sauté for 4 minutes or until mushrooms are lightly browned. Stir in broth, wine, cornstarch, salt, and pepper; cook 2 minutes or until sauce is slightly thickened. Remove from heat; add butter, stirring until butter melts.
Make sure the sauce is bubbling as it cooks so the cornstarch can effectively thicken it. A couple of tablespoons of butter round out the flavors and add richness to this sauce—stir it in off the heat so it blends in and emulsifies.