Cover elbow macaroni with water and cook to package directions, drain and reserve 1/2 cup of the water the pasta was cooked in. Return the pasta to the pan that it was cooked in.
In the meantime, in a saucepan, melt the butter. Add flour and stir for two minutes. Slowly add the milk and stir constantly until thickened. Use the pasta water to bring to consistency that you like. Add cheese and stir until it melts. Stir in the Dijon mustard until blended and pour over the macaroni.