Add onion and mushrooms and cook until tender, about 10-15 minutes, stirring occasionally.
Add minced garlic and sauté for 30 seconds.
Add flour and stir to coat the vegetables. Sauté for two minutes.
Whisk in the beef broth, red wine and thyme.
Bring the liquid to a boil, then reduce the heat to medium low and simmer until the sauce begins to thicken, about 10-15 minutes more, stirring occasionally.
Season the sauce with salt and pepper.
Video
Notes
Storage. Mushroom sauce should be transferred into an airtight container and stored in the refrigerator within two hours. Consume within four days.Freezing. Transfer the cooled mushroom sauce to a freezer bag. Remove as much excess air as possible and seal. Lay flat until frozen. Thaw before using. Consume within three months.Reheating. Reheat the thawed mushroom sauce in a saucepan over medium heat until heated through, about 10 minutes.