Stovetop mac and cheese

1. In a medium saucepan, melt butter over medium; add onion and cook until softened, about 4 minutes. Add flour and 1/2 teaspoon salt; cook, stirring frequently, until mixture is pale golden, has a slightly nutty aroma, and is the texture of cooked oatmeal, about 5 minutes.
Whisking constantly, pour in 2 cups milk; add 2 more cups milk and cayenne and whisk until smooth. Cook mixture, stirring constantly along bottom of pan, until boiling, 6 to 7 minutes. Reduce heat to low. Simmer gently, stirring occasionally, until sauce thickens and does not feel grainy when a small amount is rubbed between two fingers, about 10 minutes.
Meanwhile, in a large pot of boiling salted water, cook pasta until al dente; drain thoroughly and return to pot. Remove sauce from heat and let cool 10 minutes. Quickly whisk in cheese, mustard, and Worcestershire. Pour cheese sauce over pasta and stir to coat.