Melt butter in a medium saucepan over medium high heat. Add the flour and cook stirring for 1 minute. Whisk in the milk slowly, then whisk in the Dijon mustard, bring to a simmer and cook for 5 minutes until thick. Stir in the cheeses until completely melted. Then stir in the pasta. Spread into an even layer in an 8”x10” baking dish.
In a separate bowl, mix together the cracker crumbs with the parmesan cheese.
Top the mac and cheese with the cracker and parmesan mixture and bake about 20 minutes, until edges are bubbling. Remove from oven and let sit for 5 minutes before serving.