In saucepan, melt 2 Tablespoons of butter over medium-high heat. Add mushrooms and cook until golden and excess mushroom liquid evaporates, about 10 minutes.
Add garlic and remaining butter (4 tablespoons). Cook for about 1 minute or until butter melts. Increase heat to high.
Whisk in the flour, continually stirring to remove lumps. Add about 1/2 cup of beef stock, whisking until incorporated.
Pour in the remaining broth and whisk until well combined.Continue cooking on high heat until gravy comes to a boil.
Reduce heat to medium-low, stirring often, simmer until thickened, about 15-20 minutes depending on your preferred thickness. Season with salt and pepper, to taste (the saltiness and flavors will concentrate some when it reduces down).
Serve on your favorite meatloaf, mashed potatoes or what ever you enjoy.