Combine all ingredients except the basil in a bowl. Mix with your hands to fully combine, making sure not to over-mix so that the meatballs do not get tough. Use a tablespoon to scoop the mixture and roll it into balls. Place on parchment paper and bake for 40 minutes, turning halfway through cooking. Top with chopped basil.
Notes
Grate the onion to avoid large pieces in the meatballs.
Do include the liquid produced by the onion! It makes the meatballs super flavorful.
Grate your cheese fresh off the block to get the best melting quality.