In a medium saucepan, melt the butter. Add the onion, garlic and thyme sprig and cook over moderate heat, stirring a few times, until the onion is softened, about 7 minutes. Add the beans and broth and simmer over moderately high heat until the broth is reduced by half, about 5 minutes; discard the thyme sprig. Puree the bean mixture in a blender. Season the puree with salt and pepper and serve hot.