Preheat oven to 400 degrees F. Lightly grease a 2-quart square baking dish; set aside. Scrub potatoes; cut in 1-inch pieces. In a large saucepan cook potatoes in enough boiling, lightly salted water to cover for 12 to 15 minutes or until tender; drain.
In a 12-inch skillet melt butter over medium heat. Add garlic and thyme; cook and stir for 1 minute. Add potatoes; coarsely mash with a potato masher. Stir in buttermilk, salt, and pepper. Fold in Fontina cheese, 1/2 cup of the Parmesan cheese, and the blue cheese. Transfer mixture to the prepared baking dish, spreading evenly.
In a small bowl combine the remaining 1/2 cup Parmesan cheese, the panko, and Italian seasoning. Drizzle with oil; toss gently to coat. Sprinkle evenly over potato mixture.
Bake, uncovered, about 20 minutes or until mixture is bubbly and top is golden brown. If desired, sprinkle with parsley.