Melt butter in a 3-quart saucepan over medium heat. Add onion; cook and stir until softened and translucent, 3-5 minutes.
Add celery, carrots, broth and rice. Bring to boil; reduce heat and simmer 10-12 minutes, until rice is tender, but still has a little bite.
Meanwhile, zest lemon into a small bowl and add minced parsley and garlic; stir to combine. Juice lemon half into separate small bowl. Set both bowls aside.
When rice is tender, add turkey meat; cook and stir until heated through, 2-3 minutes. Stir in reserved lemon juice and season to taste with salt and freshly ground black pepper. Ladle soup into bowls and garnish with parsley mixture. Serve.
Homemade turkey stock (instructions in post) or chicken stock make this soup even better!
Rinse your rice if you want to remove the powdery surface starch that can thicken the soup.