In a large pot, over medium-high heat, bring water to a boil. Add the pasta and cook until al dente. Drain and set aside.
Make the sauce:
Once the pasta is cooked, start on the cheese sauce.
Warm 1 cup of the milk in a saucepan over medium heat.
In a medium bowl, whisk together the remaining ½ cup of milk and the 2 tablespoons of flour until there are no lumps and set aside.
When the milk in the pot is warm, and you begin to see steam rising from the milk, whisk in the milk-and-flour mixture. Continue whisking until the milk thickens slightly to the consistency of heavy cream, for about 2 to 3 minutes.
Turn the heat down to low and add the shredded cheese into the milk. With the whisk, stir to combine. Add in the salt and dijon mustard. Continue to stir until all the cheese has melted and the sauce is creamy. Turn off the heat.
Combine:
Add the pasta to the sauce and stir until it’s evenly coated.
To pack in a thermos
Preheat the thermos container with boiling water. This step is optional, but it helps.
Heat the mac and cheese in the microwave or stovetop to a piping-hot temperature.
Drain the water from the thermos.
Fill the thermos with the very hot mac and cheese.
Close the thermos container and pack it inside a lunch bag with your choice of snacks.
Include an ice pack to keep the other ingredients chilled, if desired.