Heat the oil in a medium skillet over medium heat. Once the oil is shimmering, add the onion. Sauté until the onions are softened and golden-brown on the edges, about 5 minutes. Spoon the onions into a large mixing bowl and cool until the onions are warm.

To the bowl of onions, add 1 cup (65 grams) of Panko breadcrumbs, ground chicken, garlic, eggs, Parmesan cheese, Italian seasoning, Kosher salt, and black pepper. Mix the meat mixture with a rubber spatula until it's well-combined. Don't over-mix or the meatballs will be dense! If the mixture is very wet and hard to roll, add another 1/4 cup (15 grams) of breadcrumbs.

Use a #60 scoop to divide the meat mixture into 1 tablespoon portions. Then dampen your hands with a little water (this keeps the meat from sticking), and roll each portion of meat into a loose ball. You should have about 60-64 small meatballs.

Grease the basket of an air fryer with neutral oil or cooking spray. Place as many meatballs as you can in a single layer, leaving a little space between each one. Air fry the meatballs at 375°F (190°C) for 6-7 minutes, tossing them halfway through, until they are golden-brown and reach 165°F (74°C) in the centers. Repeat with the remaining meatballs until they're all cooked.

To keep the cooked meatballs warm, you can place them on a sheet pan and keep them warm in a low oven for up to 1 hour. Or, add them to a pot or slow cooker of sauce over low heat.