Place the ground chicken in a large mixing bowl and grate the 1/4 onion over it so the juices fall into the bowl.
Add the crushed garlic, grated Parmesan, panko, chopped parsley, 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 evenly combined.
Lightly oil your hands and roll tablespoon-sized portions of the mixture into meatballs, then set them aside on a plate.
Heat 2 tablespoons olive oil in a large high-sided skillet or shallow casserole over medium heat.
Lightly fry the meatballs for about 2 minutes, turning to brown on most sides, then remove them from the pan and set aside.
Add the chopped 3/4 onion, carrot, and celery to the pan and cook for 3–4 minutes until softened and lightly browned.
Stir in the passata, chicken broth, 2 tsp Italian seasoning, 1 tsp sugar, and 1 tsp salt and bring the mixture to a simmer.
Add the orzo, stir to combine, then return the browned meatballs to the pan, nestling them into the sauce.
Reduce heat to low, cover with the lid, and simmer gently for 10 minutes.
Remove the lid and cook for an additional 2–3 minutes until the orzo is tender and the sauce has thickened.
Serve hot topped with extra grated Parmesan and fresh basil leaves.