Celery Root & Potato Gratin

1 cup chicken broth
2 cups peeled and thinly sliced celery root
2 cups peeled and thinly sliced white potato
1/2 cup peeled and thinly sliced yellow onion
1/2 cup skim milk
1 tablespoon butter (or margarine)
1 tablespoon flour
1/4 teaspoon salt
1/8 teaspoon ground mustard
1/2 cup shredded 2% cheddar cheese
1/2 cup crumbled reduced-fat blue cheese

Preheat oven to 400F. Spray a baking dish with cooking spray.

In a large saucepan, bring the chicken broth to a boil over medium high heat. Add the celery root, potato and onion, reduce heat to medium-low heat, cover and simmer 10 minutes. Transfer the vegetables from the pan to the prepared baking dish with a slotted spoon. Pour 1/2 cup of the cooking liquid from the saucepan into a measuring cup. Add the milk to the cup. Set aside.

Melt the butter in the saucepan. Add the flour and whisk to combine. It will form a pasty mixture. Tap the whisk on the side of the pan to release any of the butter and flour mixture that gets stuck in it. Cook one minute. Slowly pour in the chicken broth and milk mixture, stirring constantly with the whisk, until the sauce is thickened and creamy. Add the salt, mustard, and cheddar cheese, and stir until the cheese has melted and combined with the sauce.

Pour the cheese sauce evenly over the vegetables. Sprinkle the top with the blue cheese. Bake uncovered for 20 minutes or until vegetables are tender when poked with a fork.