In a large mixing bowl, combine ground beef, ground pork, breadcrumbs, Parmesan cheese, parsley, egg, garlic powder, onion powder, salt, and black pepper. Mix until well combined. Shape the mixture into meatballs, about 1 to 1.5 inches in diameter.
Optional: For a firmer meatball, you can bake the meatballs on a baking sheet at 400°F (200°C) for 15 minutes before adding them to the crock pot.
In a skillet over medium heat, add olive oil and sliced onions. Cook the onions until caramelized, about 15-20 minutes, stirring occasionally.
Once the onions are caramelized, transfer them to the crock pot. Add beef broth, Worcestershire sauce, thyme, and sugar. Stir to combine the sauce ingredients.
Gently add the prepared meatballs into the crock pot, making sure they are submerged in the sauce.
Cover the crock pot and cook on low for 6 hours or on high for 3 hours until the meatballs are cooked through.
If using, sprinkle Swiss cheese over the meatballs in the last 15 minutes of cooking, allowing it to melt.
Serve the meatballs hot with some sauce over cooked rice, pasta, or on their own as an appetizer.