In a large bowl, combine ground chicken, breadcrumbs, egg, minced garlic, chopped parsley, salt, and pepper. Mix gently until just combined.

Use your hands to form small meatballs, about 1 inch in diameter, and set them aside on a tray.

In a large pot, heat a splash of olive oil over medium heat. Add the chopped onion, carrots, and celery, cooking for about 5-7 minutes until softened.

Stir in the minced garlic and cook for another minute until fragrant.

Pour in the chicken broth along with the dried thyme and bay leaf. Bring the mixture to a gentle simmer.

Carefully add the meatballs to the simmering broth. Cook for about 10-15 minutes, until the meatballs float to the surface.

If you're adding spinach or kale, toss them in during the last couple of minutes of cooking.

Discard the bay leaf and ladle the soup into bowls. Allow it to cool slightly, then serve.