Melt butter in a large skillet over medium heat. Add the onion and cook, stirring, until soft, about 4 minutes. Add the carrot, celery, and potato, and cook, stirring, until soft, about 6 minutes. Stir in the flour, then add the broth and bring to a boil. Add the greens and chicken, and simmer until thickened, about 2 minutes. Season to taste.