Preheat oven to 400ºF and generously butter a 3-quart (9 x 13-inch) casserole dish. Layer ½ of the potato sliced into bottom. In a medium-size saucepan over medium heat, melt butter and sauté onion with garlic for 3 minutes. Using a slotted spoon, transfer onion mixture over the potatoes in the prepared casserole dish; set aside.
Return saucepan to stovetop and pour cold milk. Bring to a simmer and add cheese, stirring until cheese has melted completely. Pour cornstarch mixture and stir until sauce thickens. Pour ½ cheese sauce over onion mixture and season with black pepper and cayenne. Add the remaining potatoes and pour the remaining sauce; season with black pepper and cayenne once again. Cover dish with greased aluminum foil and bake 1 ½ hours in the preheated oven. Remove foil for the last 15 minutes for golden brown top.
Note: Sprinkle with Panko® for extra crispiness or use green onions and, crumbled and cooked bacon.