Now, a gratin is basically just a casserole constructed out of thin slices of potato layered with, well, whatever you salvaged from the refrigerator. Now, medium starch potatoes like these Yukon Golds are really perfect gratin fodder because they'll bind together when they cook without losing their consistency. So, we're just finishing up the 4th layer of the gratin. Great. Now, before we put on any other ingredients we're going to add some dairy and that's going to enrich the flavor. It's also going to aid the consistency, help the whole thing to bind together.