Chef John's Italian Meatballs

Cover a baking sheet with foil and spray lightly with cooking spray.
Soak bread crumbs in milk in a small bowl for 20 minutes.
Heat olive oil in a skillet over medium heat. Cook and stir onions in hot oil until translucent, about 20 minutes.
Mix beef and pork together in a large bowl. Stir onions, bread crumb mixture, eggs, parsley, garlic, salt, black pepper, red pepper flakes, Italian herb seasoning, and Parmesan cheese into meat mixture with a rubber spatula until combined. Cover and refrigerate for about one hour.
Preheat an oven to 425 degrees F (220 degrees C).
Using wet hands, form meat mixture into balls about 1 1/2 inches in diameter. Arrange onto prepared baking sheet.
Bake in the preheated oven until browned and cooked through, 15 to 20 minutes.