Add the onion, scallions, celery, garlic, thyme and cayenne to the pot and cook over low heat until the vegetables are softened, about 10 minutes. Add the stock and bring to a boil. Add the mixed greens and cabbage in large handfuls, stirring until theyjre wilted before adding more. Season the gumbo with salt and pepper. Cover and simmer over low heat for 1 hour, stirring occasionally, until the greens are all very tender.