Step 3
Melt butter in a large skillet over medium-high heat. Add onion, celery, carrots, mushrooms, and thyme. Sauté 2 minutes, reduce heat to medium-low and cover. Cook 5−6 minutes with the lid on, stirring occasionally (do not allow vegetables to brown). Remove lid, sprinkle vegetables with flour. Stir until flour is completely incorporated and allow to cook 1 minute.