Heat the oil and butter in a large skillet over medium heat. Add the scallions and cook until golden, about 3 minutes. Add the radishes and cook another minute. Remove the scallions and radishes from the pan and set aside. Add the leeks, chicken broth, salt, and lemon juice and cook, stirring occasionally, until the leeks are softened, about 5 minutes. Add the parsley, scallions, and radishes and toss well.