Preheat the oven to 400 degrees. Line a large baking sheet with parchment paper or foil.

In a large bowl, combine the beef, breadcrumbs, egg, steak sauce, ketchup, mustard, onion powder, garlic powder, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mix well and shape the mixture into 16 to 20 meatballs. Transfer the meatballs to the prepared pan and bake for 16 to 20 minutes, until cooked through (timing will depend on the size of your meatballs; cooked through is 160 degrees).

Meanwhile, melt 1 tablespoon of the butter in a large skillet over medium-high heat. Add the mushrooms and cook for 3 to 5 minutes, until soft and releasing juice. Transfer the mushrooms to a plate, reserving the juice in the pan (if there is any).

Melt the remaining 2 tablespoons of butter in the same pan over medium heat. When the butter has melted, whisk in the flour until smooth. Whisk in the beef broth, tomato paste, and Worcestershire sauce and bring to a simmer, whisking up any browned bits from the bottom of the pan. Simmer for 3 to 5 minutes, until thickened. Return the mushrooms to the pan with an accumulated juices on the plate.

When the meatballs have finished baking, add them to the mushroom gravy. Cook for 1 minute to heat through.

Top the meatballs with parsley (if using) and serve.