Preheat oven to 425°F. Line a rimmed baking sheet with foil and set aside.
Combine all ingredients, except spaghetti sauce and mozzarella, in a large bowl. Combine (with a spoon or your hands) until completely combined and evenly mixed.
Using a small cookie scoop (approx. 1 Tbsp in volume), scoop the meatball mixture into balls and roll to smooth with your hands. Place on the prepared baking sheet, almost but not quite touching.
Bake in the preheated oven for 15-18 minutes, or until cooked through (if you press down on them, they will feel firm, not squishy).
Remove from oven and transfer meatballs (with a slotted spoon) to an 8x8 baking dish. Cover with spaghetti sauce and add mozzarella on top (if using). Put back in the oven and bake, uncovered, until sauce is heated through and cheese is melted and slightly browned, about 5 minutes.
Serve hot with a side salad. Leftovers can be stored in an airtight container for up to a week.