In a mixing bowl, combine the ground chicken, finely chopped onion, crushed garlic cloves, grated parmesan cheese, panko breadcrumbs, chopped parsley, egg, Italian seasoning, salt, and black pepper. Mix until just combined, being careful not to overmix. Form the mixture into small meatballs, about 1 inch in diameter.
Heat the olive oil in a large pot or Dutch oven over medium heat. Once hot, add the meatballs in batches, searing them on all sides until they are golden brown. This should take about 5-7 minutes. Once browned, remove the meatballs from the pot and set them aside.
In the same pot, add the finely chopped onion, carrot, and celery. Sauté for about 5 minutes, until the vegetables are softened and the onion is translucent. Stir occasionally to prevent sticking.
Pour in the passata or marinara sauce and chicken broth. Stir in 2 teaspoons of Italian seasoning, sugar, and 1 teaspoon of salt. Bring the mixture to a gentle simmer.
Add the orzo to the pot and stir well. Return the meatballs to the pot, nestling them into the sauce. Cover the pot and let it simmer for about 10-12 minutes, or until the orzo is tender and has absorbed most of the liquid. Stir occasionally to prevent sticking.
Once the orzo is cooked, remove the pot from heat. Serve the One Pot Chicken Meatball Orzo in bowls, garnished with extra grated parmesan and fresh basil leaves.