Mix together ground chicken, onion, and garlic. In a small bowl combine breadcrumbs, dill, chili flakes, salt, and pepper, mix to combine, add half & half to moisten breadcrumb mixture, use a fork to combine. Add breadcrumb mixture to chicken mixture, mix, then form into 1 inch balls. Put in fridge until ready to add to broth.
Heat 2 tablespoons EVOO in soup pot. Add carrots, celery, and leek. Cook until soft, about 4 minutes. Add broth and bring to boil. Add meatballs to boiling broth, reduce to simmer, cover, and cook 15 minutes.
Divide watercress in bowls, ladle soup over top. Serve.