1. Heat oven to 350 degrees F. Coat 2-1/2quart shallow casserole with nonstick cooking spray.
2. Cook elbow macaroni following package directions. Drain.
3. Whisk flour and 1 cup of the milk in large saucepan until smooth. Stir in remaining 2-1/4 cups milk, salt and nutmeg; cook, stirring constantly, over medium heat until thickened and bubbly, 2 minutes. Remove from heat. Stir in cheeses and mustard. Stir in macaroni. Turn into casserole.
4. Bake in 350 degree F oven 30 minutes until lightly browned and bubbly.