Make the meatballs: Mix beef, onion, egg, breadcrumbs, Worcestershire sauce, salt & pepper. Form into small balls.
Brown them: In a skillet, sear until nicely golden on all sides. Remove and set aside.
Make the gravy: In the same pan, melt butter, add mushrooms, and sauté until soft. Sprinkle flour, stir well, then slowly pour in beef broth while whisking. Add mustard.
Simmer: Return meatballs to the pan and simmer for 10–15 minutes until the sauce thickens and the meatballs are fully cooked.
Serve: Over creamy mashed potatoes or buttered egg noodles — pure comfort!
