Potato Gorgonzola Gratin

2 tablespoons butter
2 1/2 tablespoons flour
1 teaspoon chopped, fresh thyme
2 1/2 cups fat free milk
3/4 cups(3 ounces) crumbled Gorgonzola or other blue cheese
1 1/2 teaspoons salt
1/4 teaspoon freshly ground black pepper
3 pounds baking potatoes, peeled and cut into 1/8-inch slices
1/3 cup grated Parmigiano-Reggiano cheese

Preheat oven to 375 degrees
Melt butter in a small saucepan over medium-high heat.  Add flour, and cook 2 minutes, stirring constantly with a whisk.  Stir in the thyme.  Gradually add milk, stirring with a whisk, cook over medium heat until slightly thick (about 3 minutes), stirring constantly.  Stir in salt and pepper.  Remove from heat.
Arrange one-fourth of potatoes in bottom of a 13 x 9 baking dish coated with cooking spray.  Spoon about 3/4 cup sauce over potatoes.  Repeat layers twice, arrange remaining potatoes over sauce.  Sprinkle with Parmigiano-Reggiano.  Cover and bake 375 degrees for 30 minutes.  Uncover and bake an additional 40 minutes or until potatoes are tender.  Remove from oven, let stand 10 minutes before serving.