Preparation

1. Heat 2 tablespoons olive oil in a large pot or Dutch oven over medium-high heat. Add onion, carrot, celery and cook, stirring occasionally, until onion is translucent, 4 to 5 minutes. Add garlic and cook for 1 minute.
2. Add broth, cover bring to a boil. Add orzo, oregano and salt; cover and cook, stirring occasionally, until the orzo is just tender, about 9 minutes.
3. Stir in meatballs and spinach; cook until the meatballs are heated through and the spinach is wilted, 2 to 4 minutes. Sprinkle with cheese if desired.
4. For meatballs: Position a rack in the oven 8 inches from the heat source, preheat broiler to high. Lightly coat a rimmed baking sheet with cooking spray.
5. Combine breadcrumbs and milk in a medium bowl; let stand 10 minutes.
6. Stir egg, cheese, parsley, garlic and pepper into the breadcrumb mixture. Break sausage into small pieces. Add the sausage and chicken to the breadcrumb mixture, gently mixing until combined.
7. Form the mixture into 24 meatballs, about 1 tablespoon each. Arrange the meatballs on the prepared baking sheet. Broil until an instant-read thermometer inserted in the center of a meatball register 165 degrees, 9 to 10 minutes.