Make the panade: Stir the panko and milk together in a large bowl and let it sit for 5 minutes, until the crumbs soak up the milk and turn into a soft paste.

Mix the meatballs: Add the ground beef, ground pork, egg, parmesan, garlic, parsley, Italian seasoning, salt, pepper, and onion powder to the bowl. Mix with your hands just until combined. Overmixing packs the meat too tight and makes the meatballs tough.

Roll: Scoop and roll the mixture into 1 1/2-inch balls, about 20 total. A cookie scoop keeps them even so they cook at the same rate.

Preheat the air fryer: Preheat your basket air fryer to 400°F for about 3 minutes so the meatballs start browning the second they go in.

Air fry: Arrange the meatballs in a single layer with space between them, working in two batches if needed so they aren't crowded. Air fry at 400°F for 10 to 12 minutes, shaking the basket at the halfway point, until browned all over and an internal temperature of 160°F. Air fryer models vary, so trust the thermometer over the clock.

Sauce and serve: Serve as is, or add the warm meatballs to a pot of marinara and simmer for a few minutes to coat before serving.