Prepare the Onion Mixture: In a skillet, heat a tablespoon of olive oil over medium heat. Add the chopped onion and cook until soft and caramelized, about 10-12 minutes. Add the minced garlic for the last 2 minutes of cooking. Remove from heat and let it cool slightly.
Make the Meat Mixture: In a large mixing bowl, combine the ground beef, ground pork, breadcrumbs, grated Parmesan, egg, dried thyme, salt, black pepper, and the cooled onion-garlic mixture. Mix until just combined – avoid overmixing to maintain tender meatballs.
Form the Meatballs: Preheat your oven to 400°F (200°C). Shape the mixture into meatballs, about 1 inch in diameter, and place them on a baking sheet lined with parchment paper.
Bake the Meatballs: Bake the meatballs in the preheated oven for 20-25 minutes, or until they are cooked through and browned.
Prepare the Sauce: While the meatballs are baking, in the same skillet used for onions, add the beef broth and balsamic vinegar. Bring to a simmer and allow it to reduce slightly for about 5-7 minutes.
Combine Meatballs and Sauce: Once the meatballs are done, carefully add them to the skillet with the sauce, turning them to coat well. Let them simmer together for an additional 5 minutes to soak in the flavors.
Serve: Transfer the meatballs to a serving dish, pour the sauce over them, and garnish with fresh parsley.