Preheat the oven to 400°F.
In a large bowl, combine ground chicken, beaten egg, parmesan cheese, salt, pepper, milk, ½ cup of breadcrumbs, garlic, and onion. Stir together, being careful not to overwork the ingredients.
Greasing hands beforehand and working one at a time, form the ground chicken mixture into your desired size of meatballs. (Ours were golf ball size, or about 1½ ounces each.)
Roll each meatball in the remaining breadcrumbs until evenly coated and set aside.
In a large oven-safe skillet over medium-high heat, add oil. Once hot, add the meatballs and cook until browned on all sides, about 2-3 minutes per side. They will not be completely cooked through at this point. (You may need to work in batches; don't overcrowd the pan.) Remove the pan from heat and transfer the meatballs to a plate and set aside.
After wiping down the skillet, add the marinara sauce. Nestle the meatballs into the sauce. Top with mozzarella cheese.
Bake 15-18 minutes or until the meatballs reach an internal temperature of 165°F and the cheese is melted.
Garnish with parmesan cheese and chopped basil.