In a large bowl, combine ground beef (or lamb), spinach, garlic, breadcrumbs, Parmesan cheese, egg, salt, black pepper, and Italian seasoning. Mix gently until just combined.
Scoop 1–2 tablespoons of the mixture and flatten it in your palm. Place a cube of mozzarella in the center, wrap the meat around it, and roll into a ball.
Heat olive oil in a large skillet over medium heat.
Cook meatballs in batches, turning occasionally, for 4–5 minutes per side until browned and fully cooked through.
Serve warm with marinara sauce, pasta, or vegetables. Garnish with extra Parmesan or fresh basil if desired.