Preheat oven to 35o degrees F. and grease 9×13-inch baking dish with oil.
Place beef, egg, onion, bread crumbs, Parmesan, milk, garlic, Worcestershire, mustard, hot sauce, salt, pepper and garlic salt into a large mixing bowl, stirring until just combined.
Take 1/4 cup beef mixture and press into center with your thumb to make room for cheese ball.
Place cheese in middle then with hands enclose the cheese with the beef, making sure the cheese is completely covered.
Place meatballs into prepared baking dish and bake for 30 to 35 minutes, until cooked through.
Serve over top your favorite pasta or inside a warm roll for delicious meatball sandwich.
I think everyone has their own homemade meatball recipe, I don't usually put mustard in mine, but might try that next time! I also use regular bread crumbs and no milk, but to each his own in this case I think. I also have not seen those cute little balls of mozzarella, so I just cut small squares from a brick.