Preheat oven to 200°C (400°F). Line a baking tray with parchment paper or lightly oil a baking dish.
Mix the meatball base: In a large bowl, combine the ground meat, chopped spinach, garlic, breadcrumbs, Parmesan, egg, and seasonings. Mix well until evenly combined (don’t overwork the mixture).
Stuff the meatballs: Take a heaping tablespoon of the mixture and flatten it in your palm. Place a cube of mozzarella in the centre and shape the meat around it to seal completely. Roll into a ball.
Optional browning step: Heat olive oil in a skillet over medium-high heat. Sear the meatballs for 2–3 minutes until browned on the outside. This adds flavour but can be skipped if baking directly.
Bake the meatballs: Place the stuffed meatballs on the prepared tray. Bake for 18–20 minutes or until fully cooked.
Optional sauce finish: Warm marinara sauce in a pan. Add the cooked meatballs and let simmer for 5 minutes before serving.