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 spatul, 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