Cook your pasta according to the directions on the package; cook until al dente. When done, drain and set aside. While the pasta cooks, start your cheese sauce.
Melt the butter in the skillet over medium-high heat. Once melted, gradually whisk in the flour until the mixture turns into a paste (at this point, it's a roux).
Slowly whisk the milk into the mixture. It'll start to bubble and thicken. Bring the heat down to medium.
Sprinkle the grated cheese into your sauce and stir often until the cheese melts, turning the mixture into a perfectly silky cheese sauce. Then, stir in the Dijon mustard. Add some sea salt and pepper, to taste.
Add the drained pasta to the skillet and toss to coat.
Sprinkle the parmesan over the noodles and place the skillet under the broiler for a few minutes, until the top crisps to a slight golden color. Watch it closely so it doesn't burn.
Remove the skillet from the oven and let it sit for a few minutes before digging in.