Do not wash the skillet. Place it back over medium-high heat and add the butter. When the butter is melted, and foaming add the mushrooms.
Cook the mushrooms for 5 minutes, stirring occasionally. The mushrooms will release a good amount of liquid; let most of it cook out before adding the flour.
Lower the heat to medium and sprinkle the flour over the mushrooms, stir well to combine. Cook for 1½-2 minute to get the raw flour taste out, stir often.
Slowly add the beef broth to the mushroom mixture while stirring gently and scrapping the bits off the bottom of the pan.
Stir in the Worcestershire sauce. Raise the heat to medium-high and bring the mushroom sauce to a simmer.
Add the beef tips back to the skillet (along with any juices collected on the plate). When the sauce comes to a simmer again, lower the heat to medium-low and cook, uncovered, for 15-20 minutes or until the gravy thickens. Stir occasionally and keep the sauce at a simmer, raise or lower the heat if needed.
If the sauce is not thickening cook it a bit longer. But it will thicken further as it cools. Keep it in mind so it doesn’t go too far.
Taste the mushroom sauce and add salt if needed. As a reference we did not add extra to ours.
Serve the beef and mushrooms sauce on a bed of mashed potatoes and garnish with a sprinkle of chopped parsley, if desired.