“You want to keep the fat from melting and breaking down before you cook the meatballs, so keep your meat and ingredients as cold as possible. Make the mixture in a chilled bowl, and if you are adding precooked ingredients like onions, let them cool down completely before adding them in.”
Tip is from The Kitchn: https://www.thekitchn.com/6-tips-for-making-the-best-meatballs-tips-from-the-kitchn-207272