Preheat the oven to 350°F.
Line a rimmed baking sheet with parchment paper.
In a large bowl, combine the beef, pork, eggs, Parmesan and mozzarella cheeses, garlic, pepper, and Italian seasoning. Use your hands to mix the ingredients well and form them into meatballs that are large enough to fill the onion shells in step 4.
Cut the top and bottom off each onion and then separate the layers. Stuff each onion shell with a meatball. Wrap each onion ball with bacon. Place each wrapped meatball on the baking sheet.
Bake until the meatballs are cooked through, about 30 minutes.