Bring a large pot of salted water to a boil. 

Add the noodles to the pot and cook for 5-7 minutes or until al dente. 

While the noodles are cooking, prepare the broccoli. Place the broccoli pieces in a large, deep skillet or Dutch oven. Add 2 tablespoons of water to the skillet and sprinkle the broccoli with a pinch of salt. Cover the skillet.

Heat the skillet over medium/high heat to steam the broccoli. Let the broccoli steam in the skillet for 2-4 minutes or until the broccoli has turned a vibrant green color. 

Remove the broccoli from the pot and keep the broccoli warm. 

Strain the pasta and set aside. 

Heat the same skillet that you used for the broccoli over medium heat and add butter. 

Once the butter is melted, whisk in the flour and cook for 1 minute or until a paste forms. 

Slowly add the milk into the roux, whisking continuously for 3-4 minutes until the sauce begins to thicken. You want a smooth sauce with no lumps. 

Next, fold in the shredded cheese, mustard, and salt. Mix until all of the cheese has melted.

Finally, add the cooked noodles and broccoli to the sauce and mix to combine. Serve immediately.