Heat the butter over medium heat in a deep skillet. Add the mushrooms, and cook until softened, about 3 to 5 minutes.
Sprinkle in the flour, and stir until flour is incorporated. Once flour has dissolved into the mushrooms, add the sherry to deglaze the pan.
Bring to a simmer, then reduce to low, and cook until the liquid is reduced by half. The sherry will smell sweet, and there will be barely any liquid left in the pan.
Add the soy sauce and beef broth, and stir well to incorporate. Bring to a simmer again, then lower the heat, and cook until slightly thickened, about 3 to 5 minutes. Season with salt and pepper to taste.