In a nonstick skillet, heat the remaining 1/2 tablespoon of oil. Add the reserved leeks and cook over moderate heat until softened, 3 minutes. Using a heatproof spatula, form the leeks into four 2-inch rounds. Sprinkle 1 tablespoon of the cheese over each round. Cook over moderate heat until melted, about 3 minutes. Transfer the rounds to a plate and let cool until crisp.