Step 1

Boil pasta in a medium pot of boiling salted water until al dente, about 2 minutes less than per package instructions. Drain, reserving 1 cup pasta water.

Step 2

Return empty pot to medium heat and add butter. When butter melts and foams, add flour and whisk until darkened slightly, 1 to 2 minutes. Add mustard powder and whisk to combine. Slowly whisk in milk. Bring to a boil, then reduce heat to medium and cook, stirring often, until thick enough to coat the back of a spoon, 3 to 4 minutes.

Step 3

Reduce heat to low and add cheese in 3 batches, starting with American. Follow with cheddar and Parmesan. Whisk to fully combine after adding each cheese. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Add pasta, stir to coat, and serve.