Heat the oven to 400°F and line a rimmed baking sheet with parchment paper.
In a large bowl, combine the ground chicken, panko, Parmesan, egg, garlic, Italian seasoning, salt, pepper, and red pepper flakes. Mix with your hands just until combined — overworking makes meatballs tough (stop as soon as you don't see dry crumbs).
Roll into 18 meatballs, about 2 tablespoons each, and arrange on the baking sheet. Brush or drizzle with the olive oil.
Bake 18-20 minutes, until golden and the internal temperature reads 165°F on an instant-read thermometer (the USDA safe temp for ground poultry — no guessing).
While the meatballs bake, warm the marinara in a large oven-safe skillet over medium heat. Nestle the baked meatballs into the sauce, top with mozzarella, and broil 2-3 minutes until the cheese bubbles and spots brown. Shower with basil and extra Parmesan.