Add the remaining 1/2 tablespoon of oil to the wok. Add the beans and stir-fry until crisp-tender, about 4 minutes. Add the onion and cook until softened, 2 minutes. Add the pepper and leek and cook, stirring occasionally, until the leek is lightly browned, about 3 minutes. Add the hoisin-vinegar sauce to the wok and cook just until bubbling. Return the lamb to the wok and stir-fry until heated through. Transfer to a platter and serve.