Preheat the oven to 350°F. Spray a 9" x 13" inch casserole pan with cooking spray.
Bring a large pot of salted water to a boil over high heat and cook macaroni according to instructions on the box. Do not overcook. Drain well and keep warm.
In the same pot, melt the butter on medium heat and stir in the flour. Mix with a wire whisk and slowly add milk, mustard, salt and pepper.
Add both shredded cheeses but reserve 4 ounces for the top. Mix well until the cheese sauce is smooth. Remove from heat.
Add macaroni and mix well. Next mix in the beaten eggs.
Pour mixture into the casserole dish.
Bake until it's bubbling around the edges and cheese on top is starting to brown, about 35 minutes. Let sit for a few minutes before serving.