Place your breadcrumbs into a large bowl, and add the milk to them to soak for 5 minutes. Add in your ground beef with, thyme, Italian seasoning, salt, pepper, onion, garlic, parmesan cheese, and the egg.

Using your clean hands, gently mix the beef together with the ingredients until well-combine. Scoop out 2 tablespoon-size portions of the meat mixture (I like to use my cookie scoop for this), and place onto a tray or large platter to hold.

Melt 2 tbsp butter into a large cast iron skillet over medium-high heat, and add the meatballs in batches. Sear and brown on all sides for about 8 to 10 minutes. Place the meatballs onto a tray or sheet pan to hold while you brown the rest of the meatballs.

Preheat oven to 400.

In the same skillet as you browned the meatballs, add another 2 tablespoons of butter if needed. Once hot and melted, add the onions, salt and pepper, and allow them to cook down and deeply caramelize for about 20 to 25 minutes, stirring frequently, until the onions become very dark, soft and jammy.

Add in the garlic and thyme, wine, balsamic, and allow it to bubble up and reduce for about 30 seconds. Then pour in the beef stock and whisk or vigorously stir to blend in and combine until smooth. Allow the French onion sauce to simmer to 5 minutes.

Nestle the browned meatballs into the onion sauce, and top them with a mozzarella cheese. Bake for 20 minutes until cheese is melted and slightly browned.