Step 1

Bring a 3.5 quart pot of water to a boil. Add a teaspoon of salt and the pasta. Cook according to the package directions for al dente. Do not over cook. Drain, and rinse with cool water. Set aside.

Step 2

In an oven-safe 10" or larger skillet, melt the butter over medium high heat. Add the flour and stir until no pockets of flour remain and the flour taste cooks out--one minute.

Step 3

Add the milk and stir to combine. Reduce heat to medium-low and add the salt, pepper and mustard powder. Stir continuously and slowly until the mixture thickens--6 to 8 minutes. Be patient.

Step 4

Once thickened enough to coat the back of your spatula, turn off the heat, add the cheeses and stir until melted. Taste for seasoning and add more salt if needed.

Step 5

Add pasta and stir to coat. You can serve from here or transfer to the broiler for 4 to 5 minutes until the top just starts to get brown and bubbly.