Prep the pan and oven: Heat the oven to 425°F (220°C). Line a rimmed baking sheet or 9x13-inch baking dish with foil and lightly oil it.
Make the panade: In a large bowl, combine breadcrumbs and milk.
Stir and let it sit for 2 to 3 minutes until the crumbs absorb the liquid.
Add flavor: Mix in the egg, Parmesan, garlic, parsley, Italian seasoning, red pepper flakes (if using), salt, and black pepper. Stir until combined.
Mix in the chicken: Add ground chicken. Using clean hands or a fork, gently mix until just combined.
Don’t overwork it.
Form the meatballs: Scoop about 2 tablespoons per meatball and roll gently. You should get about 20 to 24 meatballs. Place them on the prepared pan, leaving a little space between each.
Bake the meatballs: Bake for 12 to 14 minutes, until the meatballs are just cooked through and lightly golden on the bottom.
Add sauce and cheese: Pour marinara around and over the meatballs, then sprinkle evenly with mozzarella.
Add a little extra Parmesan if you like.
Finish baking: Return the pan to the oven for 8 to 10 minutes, until the cheese is melted and bubbling. If you want browned, blistered cheese, broil for 1 to 2 minutes at the end.
Rest and garnish: Let the meatballs rest for 5 minutes. Top with fresh basil and more Parmesan.
Serve warm.