You’ll need:

4 tablespoons unsalted butter
1 ½ cups chopped leeks
1 teaspoon chopped fresh thyme
½ cup heavy whipping cream
2  medium potatoes (russets or King Edwards, depending on your nationality and loyalty to the Crown)
1 ½ cups grated cheese (I like a combination of ½ Asiago and ½  Jarlsberg, but use whatever you have on hand.  Gruyere is wonderful for gratin.)
Salt & Pepper

Chop the leeks and sauté in butter until soft, about ten minutes.  Stir in the cream and the thyme and cook for another five minutes.  Meanwhile, peel the potatoes and cut them into thin slices. A mandoline is best for this job.

The mandoline is a wonderful kitchen tool, particularly if you’re into the uniformity of cut – which makes a real difference to evenness of cooking and an equal contribution to the aesthetics of presentation.  It’s only fault is a tendency to lurk malevolently in the darkness of a drawer, waiting to slice your fingers to ribbons when you reach unsuspectingly for the innocent baster which just happens to be cuddling up to the mandoline …

Spoon 1/3 of the leek mixture into a casserole, then a layer the potatoes, overlapping them somewhat.  Sprinkle with Asiago and Jarlsberg – or whatever cheese you’re using – add another layer of potatoes, and repeat the layers, ending with the cheese.

Very important!  Each time you lay down another layer of potatoes, season with a sprinkling of salt and pepper.

Cover the casserole with foil and bake in a 400 degree oven for 45 minutes.  Then uncover, and cook for another 20 minutes, or until the top is bubbly and crusty.

When you have taken the casserole out of the oven, let it sit for ten minutes or so – otherwise it will ooze like a bad horror movie.