In a spacious mixing bowl, combine the ground beef and ground pork. Add the finely chopped small onion, minced garlic, breadcrumbs, grated Parmesan cheese, chopped parsley, the large egg, dried thyme, salt, and black pepper. Mix thoroughly until the mixture is evenly combined.
Take about a tablespoon of the meat mixture and roll it into a ball, approximately the size of a golf ball. Place each meatball onto a parchment-lined baking sheet.
Preheat your oven to 400°F (200°C). Bake the meatballs for 20-25 minutes, checking for doneness with a meat thermometer reading 160°F (70°C).
In a large skillet, heat the olive oil over medium heat. Add the thinly sliced onion and cook, stirring occasionally for 15-20 minutes until caramelized. Stir in the balsamic vinegar in the last few minutes.
Transfer the baked meatballs into the skillet with the caramelized onions. Pour in the beef broth and let it simmer on low heat for 10 minutes.
Preheat your broiler. Transfer the meatballs and onion mixture into an oven-safe dish. Sprinkle the shredded Gruyere cheese over the top and broil for 2-3 minutes until melted and bubbly.
Notes
Serve with toasted baguette slices for dipping into the broth.