Cook pasta according to package directions; drain.
Melt butter in a large saucepan over medium heat. Whisk in flour, and cook, whisking constantly, 2 minutes. Gradually whisk in milk, salt, and dry mustard; cook, whisking constantly, 5 minutes or until slightly thickened. Remove from heat.
Add 1 1/2 cups cheese, stirring until cheese melts and mixture is smooth. Stir in pasta until well combined.
Spoon mixture into an 8-inch square baking dish coated with cooking spray. Sprinkle evenly with remaining 1/2 cup cheese.
Bake at 375° for 20 minutes or until golden and bubbly. Let stand 5 minutes before serving.
