In a large saucepan, melt the butter. Add the garlic and onion, season with salt and pepper and cook over moderately high heat, stirring frequently, until the onion is softened, about 3 minutes. Stir in the radishes, radish greens and thyme. Add the chicken stock and bring to a boil. Simmer the soup over moderate heat until the radishes are tender, about 10 minutes.