This is easy and quick because the potatoes are parboiled, so you don't have to spend an hour constantly checking the oven. It's based on a recipe posted on Bittman's blog, which I really recommend checking out. You can easily make this vegetarian by using veggie stock in place of chicken stock (or vegan by omitting the cream and using veggie stock instead of milk, but I wouldn't recommend it; in fact, I wouldn't recommend being vegan at all).
I made this alongside a roast chicken. We've pretty much perfected the roast chicken, though it was more the result of years of trying various "recipes" before finding the best one (which, as I should have known from the start, is Alice Waters'). But now I am posting this while eating the leftovers (which are actually what's in the photo above-- we forgot to photograph the fresh gratin) alongside Portugese sardines and an arugula salad. I forgot how good sardines and potatoes are together; perhaps I'll make something involving both soon. There are certainly plenty of both (and leeks, too) for very little money in Paris.
You'll need (serves 4-6):
2 enormous leeks, or 4 medium ones
3 medium yukon gold potatoes
1/4c heavy whipping cream
2c milk (lowfat's fine but nonfat's not)
1T chopped fresh thyme
1c chicken stock
Preheat oven to 400 degrees.
Quarter leeks lengthwise and clean them well. Slice thinly. Melt butter in a large pan. Add leeks, salt and pepper. Cook over low heat for 15 minutes.
In the meantime, peel potatoes and slice thinly (not too thinly-- 1/4 inch at most-- you don't want them falling apart). Put in a small saucepan with milk just to cover. Add salt and dried herbs, if you want (we use French cooking salt that has herbs in it). Simmer gently until just tender. Milk has a tendency to boil over, so be careful.
When the leeks are tender, add chicken stock and raise heat. Boil until almost dry, then add cream and stir until slightly thickened, about 1 minute. Turn off heat and let sit 1 minute to thicken. Stir in chopped thyme.
Spread 1/2 leek mixture in a baking dish. Using a slotted spatula or spoon, remove potatoes from milk and spread onto leeks. Top with the rest of the leeks. This shouldn't look dry; if it does, add a little of the milk the potatoes cooked in.
Bake until top is golden brown. Let cool 10 minutes before serving.