To a large saucepan or deep skillet over medium heat, melt the butter.
¼ cup unsalted butter
Add in the sliced mushrooms and season with salt.
16 ounces white mushrooms, salt
Simmer, stirring frequently, until wilted and liquid has evaporated, approximately 10-15 minutes. The mushrooms should be brown, but not black.
Add in the flour and cook, stirring frequently, another 5 minutes.
¼ cup all purpose flour
While briskly stirring, add in 1 cup of beef broth. It will slightly thicken and look creamy. Once incorporated, add in the remaining stock and stir until fully combined.