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.
Whisk in the flour, continually stirring to remove lumps. Add about 1/2 cup of beef stock, whisking until incorporated.
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).