Preheat oven to 375°.
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 the Gorgonzola; cook 3 minutes or until cheese melts, stirring constantly. Stir in salt and pepper. Remove from heat.
Arrange one-fourth of potatoes in bottom of a 13 x 9-inch 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 at 375° for 30 minutes. Uncover and bake an additional 40 minutes or until potatoes are tender. Remove from oven; let stand 10 minutes before serving.