Melt the butter in a large Dutch oven or deep skillet over medium heat. Add the sliced onions and cook about 10–15 minutes, stirring occasionally, until they’re soft and nicely browned.
Stir in the minced garlic, garlic powder, onion powder, smoked paprika, pepper and thyme. Cook for about 30–60 seconds.
Add the Worcestershire sauce, condensed French onion soup and beef broth. Stir well, scraping up any browned bits from the bottom.
Put the frozen meatballs directly into the sauce and bring everything to a gentle simmer.
Reduce the heat to medium-low, cover, and simmer for about 20–25 minutes, stirring every 5 minutes or so. The meatballs should be completely hot throughout.
Mix 1 Tbsp cornstarch with 1 Tbsp cold water. Stir it into the simmering sauce and cook 2–3 minutes until thickened.
While the meatballs cook, boil the egg noodles according to the package directions. Drain.
Turn the heat to low and sprinkle the mozzarella over the meatballs. Cover for 2–3 minutes, until melted.
Serve the meatballs and that wonderful French-onion gravy over the egg noodles. Sprinkle with parsley.