Preheat the air fryer. Set to 375°F (190°C) for 5 minutes.
A hot basket helps the meatballs brown right away.
Make the panade. In a large bowl, whisk the egg and milk. Stir in the breadcrumbs and let sit 2–3 minutes until it forms a soft paste. This keeps the meatballs moist.
Combine the mixture. Add Italian sausage, ground beef, Parmesan, onion, garlic, parsley, Italian seasoning, salt, and pepper.
Mix gently with your hands just until combined. Do not overmix or the meatballs can turn dense.
Form meatballs. Lightly oil your hands and roll the mixture into golf-ball-sized meatballs, about 1.5 inches wide (roughly 18–20 meatballs). Keep them uniform so they cook evenly.
Prepare the basket. Lightly spray the air fryer basket with olive oil to prevent sticking. Arrange meatballs in a single layer with a little space between each.
Work in batches if needed.
Air fry. Cook at 375°F (190°C) for 10–12 minutes, shaking the basket halfway or flipping the meatballs at 6 minutes. They’re done when browned and the internal temperature reaches 165°F (74°C).
Rest and serve. Let the meatballs rest 3 minutes to redistribute juices. Serve with warm marinara, over pasta, with zoodles, or in a toasted sub with melted mozzarella.