Preheat oven to 400°F. Line a large rimmed baking sheet with aluminum foil and spray with olive oil cooking spray.

In a large bowl, combine ground beef, Italian sausage, egg, breadcrumbs, parmesan cheese, finely diced onion, half & half, garlic, Italian seasoning, salt, and pepper. Mix gently until well combined.

Use a 2-inch cookie scoop to form 15 meatballs and place them on a plate.

Flatten each meatball into a disc about 3.5 inches wide. Place one mozzarella cube in the center, fold the edges around the cheese, and gently shape into a meatball ensuring the cheese is enclosed.

Place stuffed meatballs onto the prepared baking sheet. Repeat with remaining mixture and cheese cubes.

Bake for 20-25 minutes until browned and cooked through. Check one for doneness.

Remove from the oven and let rest for 5 minutes.

While resting, warm marinara sauce in a skillet over medium-low heat.

Serve meatballs topped with warm sauce and garnished with chopped parsley if desired.