Using tongs, transfer the cooked pasta to the garlic sauce, along with whatever water clings to it. (Reserve the pasta water in the
skillet.) Increase the heat to high, add the cheese to the
wok, and stir with a wooden
spatula or spoon and toss vigorously until the sauce is creamy and emulsified, about 30 seconds. If the sauce looks too watery, let it keep reducing. If it looks greasy, splash some more cooking water into it and let it re-emulsify. Stir in the scallions (if using), and serve immediately.