Melt 3 tablespoons of the butter in a large enameled cast-iron casserole. Add the leeks and thyme sprigs and cook over low heat until softened, about 8 minutes. Add the wine and simmer over moderately high heat for 2 minutes. Add the chicken stock, celery root, potatoes and a large pinch of salt and bring to a boil. Cover and simmer over low heat until the celery root and potatoes are tender, about 45 minutes.