Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
In a large skillet, heat a tablespoon of olive oil over medium heat. Add the finely chopped onion and cook until soft and caramelized, about 15-20 minutes. Stir frequently to avoid burning. Add the minced garlic during the last 2 minutes of cooking, then remove from heat and let cool.
In a large bowl, combine the ground beef, ground pork, breadcrumbs, egg, beef broth, soy sauce, Worcestershire sauce, thyme, black pepper, and salt. Add the cooled onion and garlic mixture and mix until well combined.
Using your hands, form the mixture into golf ball-sized meatballs (about 1.5 inches in diameter) and place them on the prepared baking sheet.
Bake the meatballs in the preheated oven for about 20-25 minutes, or until cooked through and browned.
Remove the meatballs from the oven, sprinkle the shredded Gruyère cheese on top of each meatball, then return to the oven for an additional 5 minutes, or until the cheese is melted and bubbly.
Once done, remove from the oven and let them rest for a couple of minutes.