In a large skillet, over medium heat, melt the butter. Whisk in the flour and dry mustard, and whisk constantly while the flour bubbles for a minute-- to let the flour taste cook out.
4 tbsp butter, 1/4 cup flour, 2 tsp dried mustard
Slowly stir in the half and half, about a quarter cup at a time, letting the mixture come together and thicken completely in between each addition. Let the cream sauce cook, stirring occasionally, until very thick. Reduce the heat to low.
2 1/2 cups half & half
Stir a small amount of the sauce into the beaten egg, just until smooth. Stir the tempered egg mixture into the sauce until the sauce is smooth and it's completely incorporated. Stir in about the cheeses (reserving some of the shredded cheddar for topping), until completely melted and incorporated into the sauce. Season with the Cajun seasoning (if using) and salt & pepper, to taste.
1 egg beaten, 1 cup shredded smoked gouda, 4 cup shredded sharp cheddar, 1/2 tsp Cajun seasoning, salt & pepper
Pour the pasta into the sauce and carefully toss until evenly combined. Spray a 9x13 baking dish lightly with non stick cooking spray. Transfer the mac and cheese evenly out into the prepared dish.
5 cups cooked elbow noodles
Sprinkle the remaining cheese evenly out over top of the macaroni. Bake at 350 degrees for 20-25 minutes, or until golden brown & bubbly.
Serve warm & enjoy!