Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
In a large mixing bowl, combine the ground beef, ground pork, breadcrumbs, grated parmesan cheese, eggs, salt, and pepper. Mix well until fully combined.
Shape the mixture into small meatballs, about 1 inch in diameter, and place them on the prepared baking sheet.
Bake the meatballs in the preheated oven for 20 minutes, or until cooked through and browned.
While the meatballs are baking, heat olive oil in a large skillet over medium heat. Add the sliced onions and cook, stirring occasionally, for about 15-20 minutes until they are caramelized and golden brown.
Add the minced garlic to the caramelized onions and cook for an additional 2 minutes.
Once the onions are caramelized, pour in the beef broth and Worcestershire sauce, then sprinkle in the dried thyme. Simmer for about 5 minutes until slightly thickened.
Add the cooked meatballs to the skillet, gently stirring to coat them in the onion mixture.
Top the meatballs with slices of Gruyère cheese, cover the skillet, and let the cheese melt for about 2-3 minutes.
Serve the French onion meatballs hot, spooning extra onion sauce over the top.