Boil macaroni in salted water until just tender. Reserve 1/2 cup pasta water; drain.

Melt butter in a saucepan; whisk in flour. Cook 1 minute until smooth.

Gradually whisk in milk; simmer 4 minutes until it coats a spoon.

Remove from heat. Stir in cheddar, Monterey Jack, salt, and mustard until smooth, adding reserved water if needed.

Spoon sauce into shallow bowls; mound hot macaroni on top for diners to mix.