In a small bowl, whisk together the evaporated milk, cornstarch, ground mustard, salt, pepper, shredded cheese, and chopped broccoli. Pour it over top of the elbow macaroni.
Close the lid on the HeatsBox and plug it in.
Choose these settings from the app: 185 degrees F (85 degrees C), Large portion, Cooking, 45 minutes.
Once the cooking time is over, top with additional shredded cheese, if desired.