Preheat your oven to 400°F (200°C). Line a large, rimmed baking sheet with parchment paper for easy cleanup.

In a large mixing bowl, combine the Panko breadcrumbs and the whole milk. Stir and let it sit for 5-10 minutes. The breadcrumbs will absorb the milk, creating a 'panade' which is the secret to incredibly moist meatballs.

To the bowl with the panade, add the ground beef, ground pork, grated onion, grated Parmesan cheese, chopped parsley, minced garlic, beaten eggs, kosher salt, dried oregano, black pepper, and optional red pepper flakes.

Using your hands, gently mix all the ingredients until they are just combined. Be careful not to overwork the mixture, as this can lead to tough meatballs.

Using a medium cookie scoop (about 2 tablespoons) for uniform size, roll the mixture into balls, about 1.5 inches in diameter. This should yield approximately 24-28 meatballs.

Arrange the meatballs in a single layer on the prepared baking sheet, making sure they aren't touching to allow for even browning.

Bake for 20-25 minutes. The meatballs should be beautifully browned and cooked through. An instant-read thermometer should register 165°F (74°C) in the center of a meatball.

For an extra-crispy, browned exterior, you can switch the oven to broil for the final 1-2 minutes of cooking. Watch them very closely to prevent them from burning.

Remove from the oven and let them rest for a few minutes. Serve hot with your favorite marinara sauce over spaghetti, in a sub, or on their own as a delicious appetizer.