Bring a large pot of water to a boil, add the tablespoon salt and cook macaroni according to its package until just tender. Reserve 1/2 cup pasta water, then drain.
While the pasta cooks, melt butter in a large saucepan over medium heat. Whisk in flour and cook for 1 minute without browning.
Gradually whisk in milk, then add mustard, sauce salt and pepper. Simmer gently for 4 to 5 minutes, stirring, until the sauce lightly coats a spoon.
Remove from the heat and stir in cheddar and Monterey Jack by handfuls until smooth. Avoid boiling after the cheese goes in.
Fold in macaroni and add the reserved pasta water 1 tablespoon at a time only if needed to keep the sauce flowing. Serve promptly from a warm bowl.