Place the ground chicken in a large mixing bowl and grate 1/4 cup onion over it, including the juices.
Add 2 crushed garlic cloves, 1/3 cup grated parmesan, 1/2 cup panko, 1/4 cup 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 just combined.
Lightly oil your hands and roll the mixture into tablespoon-sized meatballs, placing them on a plate while you work.
Heat 2 tablespoons olive oil in a large high-sided skillet or shallow casserole over medium heat.
Fry the meatballs in batches, turning, until lightly golden about 2 minutes per side; remove and set aside.
Add the remaining 3/4 chopped onion, 1 peeled and chopped carrot, and 1 chopped celery stick to the pan and cook 3–4 minutes until softened and lightly browned.
Stir in 2 cups passata, 1 cup chicken broth, 2 tsp Italian seasoning, 1 tsp sugar, and 1 tsp salt and bring to a simmer.
Add 1/2 cup orzo and stir to combine, then return the meatballs to the pan, nestling them into the sauce.
Reduce heat to low, cover with the lid, and simmer for 10 minutes.
Remove the lid and simmer 2–3 more minutes until the orzo is cooked and the sauce has thickened.
Serve topped with extra grated parmesan and fresh basil leaves.
