Gather all ingredients.

Cover a baking sheet with foil and spray lightly with cooking spray. Soak bread crumbs in milk in a small bowl for 20 minutes.

Meanwhile, heat olive oil in a skillet over medium heat. Add onion, cook and stir until onion has softened and turned translucent, about 5 minutes.

Reduce heat to low and continue cooking and stirring until onion is very tender, about 15 minutes more.

Gently stir beef and pork together in a large bowl. Add onions, bread crumb mixture, eggs, Parmesan cheese, parsley, garlic, salt, black pepper, Italian herb seasoning, and red pepper flakes; mix together using a rubber spatula until combined. Cover and refrigerate for about one hour.

Preheat the oven to 425°F (220°C).

Form mixture into balls about 1 ½ inches in diameter; arrange in a single layer on the prepared baking sheet.

Bake in the preheated oven until browned and cooked through, 15 to 20 minutes.