Cook pasta according to package directions. Preheat oven to 375°F.
In a medium saucepan, melt margarine or butter; stir in flour, mustard and salt. Gradually stir in milk; cook and stir until mixture thickens slightly and bubbles. Remove from heat.
Add 1 cup cheese; stir until melted. Add pasta. Spoon into a greased 8- or 9-inch square baking dish. Top with remaining 1 cup cheese.
Bake 25 minutes or until lightly browned and bubbly. Let stand 10 minutes before serving.