Boil a large pot of salted water. Add pasta and cook according to package directions, stirring occasionally, until al dente.
While pasta cooks, make the sauce: In a bowl, stir together milk, cornstarch and flour. Melt butter in a large, nonstick skillet over medium heat. Add onion and cook, stirring, until softened and golden, about 6 minutes. Stir milk mixture again, then add to onion in pan. Sprinkle with salt and pepper. Cook, stirring, until sauce is thick and creamy, about 4 minutes.
Drain pasta and add to sauce, tossing well. Sprinkle with cheese and toss again. Divide among 4 plates, sprinkle with parsley and more cheese, then serve warm.