Use a mandolin or food processor fitted with a slicing blade to get the potatoes as thin as possible. It's the key to this delicious dish. You're going to need a 2 or 3 quart gratin dish, but don't grease it.Source unknown
Adjust oven rack to middle position and heat oven to 350°. Toss cheddar, Monterey Jack, Parmesan and cornstarch in a large bowl until cheese is evenly coated.
Shingle half of potatoes in large gratin dish, sprinkle evenly with 1 cup cheese mixture, three quarter teaspoon salt, and 1/4 teaspoon pepper. Top with remaining potatoes, 3/4 teaspoon more salt and 1/4 teaspoon more pepper.
Mix cream and broth in a large measuring cup and pour over potatoes. Top with remaining cheese mixture. Bake until Golden Brown and fork inserted into center meets little resistance, 1 1/4 to 1 1/2 hours. Let cool for 10 minutes and serve.