Lower the heat to medium, sprinkle the flour over the mushrooms, stir well to combine. Cook the flour for 1½-2 minute to get the raw taste out, stir often.
→ Add the Worcestershire sauce.
→ Bring the mushroom sauce to a simmer.
→ Add the pieces of beef back to the skillet.
→ Lower the heat to medium-low and cook, uncovered, for 15-20 minutes or until it thickens.
→ Stir occasionally and keep at a simmer.