Preheat oven to 350 degrees.
Bring water to full boil in 2 quart pot. Add pasta, and cook uncovered at boil for 10 minutes. Drain and add to cooking dish.
Melt butter in pot over low heat.
Add mustard to melted butter and mix in.
Add flour to the butter, stirring to mix it thoroughly into butter mixture.
Slowly add milk to butter mixture, stirring constantly. Stir over medium low heat until mixture thickens, then take off the heat.
Add grated cheese to the milk mixture, and stir well. Reserve some of the cheese for topping the pasta.
Pour over the pasta, mix in to coat thoroughly, and sprinkle cheese on top.
Bake at 350 for 30 minutes.