In a large mixing bowl, combine ground chicken, breadcrumbs, grated Parmesan, egg, minced garlic, chopped rosemary, olive oil, salt, black pepper, and optional red pepper flakes. Gently mix with your hands until just combined, avoiding overmixing to keep the meatballs tender. Allow the mixture to rest while you prepare your workspace.

Using your hands, form the mixture into 1-inch meatballs, placing them on a baking sheet lined with parchment paper. For best results, refrigerate the meatballs for at least 30 minutes.

Preheat your oven to 400°F (200°C). Alternatively, heat olive oil in a large skillet over medium heat. Bake the meatballs for 20-25 minutes until golden brown or sear them in the skillet until cooked through.

In the same skillet, add finely chopped onion and sauté for 5-7 minutes, or until translucent. Then, stir in minced garlic and cook for an additional minute.

Add crushed tomatoes, tomato sauce, dried oregano, and basil to the skillet. Season with salt and pepper, then bring to a simmer. Cook for 10-15 minutes.

In a separate pot, bring chicken broth to a boil. Add orzo pasta and cook according to package directions, typically 8-10 minutes.

Once the orzo is tender, gently stir the meatballs into the tomato sauce for about 5 minutes to heat through.

Serve the meatballs and tomato orzo hot, garnished with freshly torn basil leaves.