Grate 1/4 onion directly into a large mixing bowl so the juices fall into the bowl, then add 1 lb ground chicken.
Add 2 crushed garlic cloves, 1/3 cup grated Parmesan, 1/2 cup panko, 1/4 cup finely chopped parsley, 1 egg, 1/2 tsp Italian seasoning, 3/4 tsp salt, and 1/2 tsp black pepper to the bowl and mix with your hands until combined.
Lightly oil your hands and roll the mixture into tablespoon-sized meatballs, placing them on a tray until ready.
Heat 2 tbsp olive oil in a large high-sided skillet or shallow casserole over medium-high heat.
Cook the meatballs in batches, browning on each side for about 2 minutes until golden but not fully cooked through, then remove and set aside.
Add the 3/4 finely chopped onion, 1 peeled and finely chopped carrot, and 1 finely chopped celery stick to the pan and cook 3–4 minutes until softened and lightly browned.
Stir in 2 cups passata or marinara, 1 cup chicken broth, 2 tsp Italian seasoning, 1 tsp sugar, and 1 tsp salt; bring to a simmer.
Add 1/2 cup orzo to the simmering sauce and stir to combine.
Return the browned meatballs to the pan, reduce heat to low, cover, and simmer for 10 minutes.
Remove the lid and cook uncovered for 2–3 more minutes until the orzo is tender and the sauce has thickened.
Serve topped with extra grated Parmesan and fresh basil leaves.