Preheat oven to 350℉
In the Medium Pot, cook the Macaroni until still a bit too firm to eat
In the Small Bowl, beat the Egg
In the Large Pot, melt the Butter
Whisk the Flour with the Butter, and cook on medium-low heat for 5 minutes
Whisk in the Milk and Dry Mustard until smooth
Cook for 5 minutes, over medium-high heat, until sauce thickens
Take ¼ cup of the sauce, and slowly whisk into the bowl with the Egg to avoid cooking it
Combine the egg mixture into the sauce, whisking until smooth
Mix in the cheese, allowing it to completely melt in the sauce
Mix in the Salt, Seasoned Salt, and Pepper
Mix the Pasta and the sauce in the Baking Dish
Bake for 20-25 minutes