In a large bowl, combine ground beef, breadcrumbs, milk, egg, chopped onion, Worcestershire sauce, salt, and pepper. Mix gently until just combined.
Form the mixture into 1.5-inch meatballs.
Heat a large skillet over medium-high heat. Add meatballs and brown on all sides, about 5-7 minutes. Remove meatballs from skillet and set aside.
Make Gravy
In the same skillet, melt butter over medium heat. Add sliced mushrooms and chopped onion. Cook until softened, about 5 minutes.
Sprinkle flour over the mushrooms and onions. Stir and cook for 1 minute.
Gradually whisk in beef broth, scraping up any browned bits from the bottom of the skillet. Bring to a simmer.
Stir in Worcestershire sauce, salt, and pepper.
Return the browned meatballs to the skillet. Reduce heat to low, cover, and simmer for 15-20 minutes, or until meatballs are cooked through and the gravy has thickened.
Serve
Serve the Salisbury steak meatballs hot, spooning plenty of gravy over them. Great with mashed potatoes or noodles.
Notes
For an extra rich gravy, you can add a splash of heavy cream at the end of cooking.