Preheat your oven to 375°F (190°C).
In a mixing bowl, combine the ground beef, breadcrumbs, milk, chopped onion, egg, Worcestershire sauce, garlic powder, salt, and pepper. Mix until well combined.
Form the mixture into meatballs, about 1.5 inches in diameter, and place them on a baking sheet.
Bake the meatballs in the preheated oven for 15-20 minutes, or until they are cooked through and browned.
While the meatballs are baking, heat olive oil in a skillet over medium heat. Add the flour and whisk for about a minute to create a roux.
Gradually whisk in the beef broth, ensuring there are no lumps, then add the soy sauce and Dijon mustard. Cook for about 5 minutes, stirring until the gravy thickens.
Once the meatballs are done, add them to the skillet with the gravy and gently stir to coat.
Simmer on low heat for about 5 minutes to allow the flavors to meld together.
Serve the Salisbury steak meatballs with your choice of sides.