Place a baking rack on the top third of the oven, and preheat the oven to 425°F. Spray a rimmed baking sheet or a 13 x 9 inch pan with nonstick spray.
Bring a large pot of water to a boil, and salt it generously. Allow the water to return to a boil and cook the pasta according to package directions. Drain the pasta.
Beat the egg into the milk until incorporated (little tip: to save washing an extra bowl, measure the milk in a liquid measuring cup, then whisk the egg in.) Melt the butter in the same pot you used to cook the pasta over medium heat. Add the flour and stir frequently for 1 to 2 minutes until the mixture turns golden.
Slowly pour in the milk mixture, stirring or whisking constantly. Turn the heat to medium-high, and stirring often, bring the milk to just barely to a simmer. Immediately reduce the heat to low and cook, continuing to stir frequently, until the mixture has thickened slightly, about 3 minutes.
Add the cheddar cheese and the Pepper Jack slowly, one handful at a time, stirring after each addition until it is melted; the cheese sauce should be very thick and quite smooth. Stir in the mustard and season with salt and pepper as you wish. Add the drained pasta to the cheese sauce and stir until well combined. Add the bacon, if using, and stir until it is well distributed.
Turn the mixture into the prepared baking pan and spread it out so that it covers the bottom of the pan evenly. Bake for 25 minutes for the sheet pan, about 30 minutes for the 13x9-inch baking pan, or until the mac and cheese is firm and the top is lightly browned in spots. Remove from the oven and let it sit for 10 minutes on a wire rack. Cut into squares with a sharp knife, and serve warm or at room temperature, using a spatula.