This potato gratin is a classic comfort food. The thinly-sliced, shingled potatoes are meltingly tender, and while theres certainly plenty of rich and creamy cheese sauce, its not so much that it drowns the potatoes. Uncovering the casserole for the final 30 minutes of baking yields a crunchy, cheesy, salty top layer thats incredibly addictive. Using two types of cheese is also key to this dish: the sharp cheddar cheese compliments the saltiness of the parmesan. Serve this casserole as a holiday side dish, or enjoy some with dinner tonight, and eat leftovers for breakfast, topped with a fried egg.