Melt the butter in a large skillet or pan over medium-high heat. Add the sliced mushrooms and cook, stirring occasionally, until they release their liquid and start to turn golden brown, about 5-7 minutes.
Add the chopped shallot to the pan and cook for another 2-3 minutes until softened and fragrant. Stir in the minced garlic and cook for just 30 seconds more.
Sprinkle the flour over the mushroom and shallot mixture. Stir constantly for about one minute to cook out the raw flour taste.
If using wine, pour it into the pan. Use a wooden spoon to scrape up all the delicious browned bits from the bottom of the pan. Let the wine simmer and reduce by about half.
Slowly pour in the beef broth while whisking continuously to prevent lumps. Add the thyme, then bring the sauce to a gentle simmer.
Reduce the heat to low and let the sauce simmer for 8-10 minutes, or until it has thickened to your desired consistency.
Stir in the chopped fresh parsley. Taste the sauce and season generously with salt and black pepper. Serve immediately.
NOTES
You can add ½ cup of whipping cream either when deglazing or during the simmer process for a creamy finish
