Add flour, stir, then gradually add beef broth, whisking to prevent lumps. Stir in Worcestershire sauce and return beef to skillet. Simmer until thickened.
Mash the cooked potatoes with butter and milk until smooth. Season with salt and pepper.
Serve mashed potatoes with generous spoonfuls of beef gravy on top.
Notes
Store leftovers in an airtight container in the fridge for up to 3 days. Add a splash of beef broth when reheating to maintain the gravy's smooth texture.