In a large bowl, gently mix the ground chicken, breadcrumbs, Parmesan, egg, garlic, parsley, salt, and pepper until just combined. Don’t overwork the meat—a light hand keeps the meatballs tender.

Shape the mixture into about 16 small meatballs, roughly 1 1/2 inches in diameter. Flatten each a touch and insert a small cube of mozzarella in the center, sealing the meat around it.

Heat the olive oil in a large skillet over medium heat. Add the meatballs in batches, browning on all sides for about 2–3 minutes per side. They don’t need to be cooked through yet, just nicely browned.

Pour the marinara sauce into the skillet, returning the meatballs to the pan if you left any out. Spoon a bit of sauce over each meatball.

Reduce the heat to a gentle simmer and cook for 8–10 minutes, or until the meatballs reach an internal temperature of 165 F (74 C) and the sauce thickens slightly.

If you prefer the cheese on top, sprinkle a little extra mozzarella. Cover the pan for a minute to melt, then serve over pasta, rice, or zucchini noodles.