Bring a small pot of water to a boil. Meanwhile, peel and dice the sweet potato into 1/2" pieces. Place in the boiling water and cook until tender, about 10-20 minutes. You can alternatively place the potato on a baking sheet and bake at 400 °F for 1 hour or until soft and then scrape out the flesh (you can really use any method at all to cook the sweet potato).
If you want a totally smooth sauce like in the photos, use an immersion blender to puree the sweet potato. A small food processor or a high-speed blender (using the smaller nut butter jar) would also work. If you don't mind a little texture, you can mash the cooked sweet potato very well until no lumps remain. I used a flat-bottomed glass and it worked fine. Measure out 1 cup (245 grams).
Bring a medium pot of water to a boil and cook the macaroni according to the instructions on the package. Drain and set aside.
While the pasta is cooking, heat the olive oil in a large saucepan over medium-high heat. Add the flour and whisk for 1 minute. Slowly pour in the milk while whisking constantly. Cook until the mixture is thick enough to coat the back of a spoon, about 3-5 minutes.
Stir in the mustard, if using, cheese, and 1 cup of mashed / pureed sweet potato. If needed, cook another few minutes until thickened.
Add in the salt, pepper and then the macaroni. Stir until totally combined. Serve hot.
Let leftovers cool, cover and refrigerate to up to 5 days.