Combine the milk and breadcrumbs: Pour the milk over the breadcrumbs in a small bowl and set aside while preparing the rest of the meatball mix. The breadcrumbs will absorb the milk and become soggy.
Whisk the egg, salt, pepper, Parmesan, and parsley: Whisk the egg in a large bowl until blended. Whisk in the salt and a generous quantity of black pepper, then whisk in the Parmesan and parsley.
Combine the egg and ground meat: Add the meat to the egg mixture. Use your hands to thoroughly mix the egg into the ground meat.
Add the onions and soaked breadcrumbs: Add the onions, garlic, and soaked breadcrumbs to the meat. Mix them thoroughly into the meat with your fingers. Try not to overwork the meat; pinch the meat between your fingers rather than kneading it.
Form the meat into meat balls: Pinch off a piece of the meat mixture and gently roll between your hands to form 1 1/2-inch meatballs. Continue shaping until all the meat is used.
To roast the meatballs in the oven: Arrange the meatballs spaced slightly apart on a baking sheet. Cook under the broiler for 20 to 25 minutes or bake at 400°F for 25 to 30 minutes. (Watch closely if cooking meatballs made with lean meat.) The meatballs are done when cooked through and the outsides are browned, and when they register 165°F in the middle on an instant read thermometer. Serve immediately.
To cook the meatballs in sauce: Bring a marinara sauce or other pasta sauce to a simmer and add as many meatballs as will fit comfortably in the pan. Cover and simmer for 30 to 35 minutes. The meatballs are done when they’re cooked through and register 165°F in the middle on an instant read thermometer. Serve immediately.