Preheat the oven to 400 °F (200 °C). Line a large baking sheet with parchment or foil.

In a large mixing bowl, combine the ground beef, breadcrumbs, egg, steak sauce, ketchup, mustard, onion powder, garlic powder, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Shape into 16 to 20 meatballs and lay them on the preheated pan. Bake for 16 to 20 minutes, or until the internal temperature reaches 160°F/71°C.

Melt 1 tablespoon butter in a large pan over medium-high heat. Cook until the mushrooms are tender, about 3 to 5 minutes. Transfer to a dish and save the pan juices.

In the same pan, cook the remaining butter over medium heat. Whisk in the flour until smooth, then gently add the beef broth, tomato paste, and Worcestershire sauce. Bring to a boil and cook for 3 to 5 minutes, until thickened. Return the mushrooms to the pan.

Add the cooked meatballs to the gravy and simmer until heated through. Garnish with parsley if desired.