Step 1: In a large bowl, gently combine ground beef, ground pork, chopped spinach, minced garlic, breadcrumbs, egg, Parmesan cheese, Italian seasoning, salt, and pepper. Mix just until combined; do not overmix, as this can lead to tough meatballs.
Step 2: Take about 1.5-2 tablespoons of the meat mixture and flatten it into a disk in your palm. Place one mozzarella cube in the center. Carefully mold the meat around the cheese, ensuring it is completely sealed within the meatball. Roll gently to form a smooth ball. Repeat with the remaining mixture and cheese.
Step 3: (Optional, for pan-frying): Heat olive oil in a large skillet over medium-high heat. Brown the meatballs on all sides, about 5-7 minutes total. Transfer to a baking sheet.
Step 4: (For baking): Preheat oven to 375°F (190°C). Arrange meatballs (either browned or raw) on a baking sheet.
Step 5: Bake for 20-25 minutes, or until cooked through and the internal temperature reaches 160°F (71°C). The cheese should be melted and gooey.
Step 6: Serve immediately with warm marinara sauce and your favorite pasta, or in a sub roll.