I've been busy cooking from Dorie's new book this month and enjoying ever bit of it. I love that I'm actually using this book to its fullest, right away. How many of us buy such beautiful cookbooks that we love that we really don't use enough? I am guilty of this as well. Since this month is all about American Thanksgiving for my foodie friends to the south, there's been a rather informal round of French Friday's with Dorie. We've been instructed to cook the 4 selections on our own schedule throughout the month of November.
The Potato Gratin is really just a more sophisticated sister of the Scalloped Potatoes that I grew up with. The idea is the same - thinly sliced potatoes cooked in the oven bathed in milk or cream - but topped with a brown, crusty Gruyere cheese topping : that's the Gratin part. What you end up with is tender, moist, cooked potatoes held together by starch and dairy proteins and a satisfying cheesy topping. Its comfort food at its best.
This flexible side dish actually makes me happy its cold, rainy or snowy outside, because that is when food like this tastes so good. Its adaptability makes it a favourite of mine since its as comfortable beside a roast chicken and a pork tenderloin as it is beside a baked salmon fillet or any manner of vegetarian dishes. 
And the fact that it can be assembled a few hours ahead of time and tucked away, on a cool patio, or even cooked before other oven ready dishes and kept on the back of the stove, make this a dinner party star. But mostly its that it tastes so so good and every person young and old will love it.
This recipe is my classic and I make it this way often. I'm insistent on the type of potato though and urge you to seek out a yellow potato for this dish, a Russet will easily fall apart and doesn't have the buttery flavour for this dish. The waxy consistency of a Yukon Gold will keep its shape when baked and will slice beautifully. If you frequent farmers markets seek out German Butterball or Sieglund for their natural butteriness. And PLEASE save those expensive Fingerlings for braising or boiling, do not waste them here! You can include other sliced root vegetables in the mix along with potatoes or layer sauteed onions or leeks in between. Even if you are short on potatoes, you likely will always have the ingredients on hand for this dish. I love to add sauteed fennel or leeks in between the potatoes. Celery root or sweet potatoes also complement the regular potatoes in a profound way. Take a look in the fridge and see what's hanging around in there. 
Tossing in woody herbs that might still be holding on outside, such as rosemary, thyme or sage make sense, but aren't necessary. And if you have recently jumped on the gourmet salt wagon lately this is one of the recipes to break out any flavoured ones you have around. I love to use my Vignalta herb salt from Italy made from fresh wild herbs grown on the the estate near where the salt in harvested. The options are endless.
Perfect Potato Gratin serves 4-6
1 3/4 pounds yellow potatoes, such as German butterball or Yukon Gold, unpeeled and sliced thin
3 cloves garlic, thinly sliced
fresh rosemary or thyme leaves, optional, but highly recommended.
fresh grated nutmeg
salt (a good time to use a flavoured salt) and pepper, white if you have it
1 cup whole milk, do not use low fat here
1/2 cup shredded Gruyere cheese
1 tbsp butter, plus more for pan
Rub a 1 1/2- 2 litre shallow stoneware or pyrex casserole or pie plate with butter. Begin layering potatoes in a single layer on the bottom of the dish, overlapping slightly, like fish scales. Season with some white pepper, a dusting of nutmeg, salt and then toss over half of the garlic slices and a few leaves of rosemary or thyme.
Add another layer of potatoes, season, the remaining garlic and herbs. Repeat until you've used all your potatoes, you should have 3 layers. Season the last layer and then pour over the milk or cream, trying to hit all the surfaces of the top layer of potatoes so they receive a dip in the milk. You may not need all the cream. It should come up about halfway up the potatoes. Top with grated cheese and then dot with pieces of butter.
Make a tent of foil and place on dish. At this point it can be put aside for 4 hours or cooked right away. Place in the oven for 30 minutes. Lift the foil and if the liquid is beginning to bubble, remove the foil, or let it cook covered another 5 minutes until bubbling. After removing foil, continue to cook about 20 - 30 minutes until the topping is nicely browned and the edges are bubbling furiously.
Let cool for 10-15 minutes before serving.
recipe notes:
- Use one of these readily available, inexpensive Japanese mandolines to sliced the potatoes in a flash! Mine cost me $18 more than 5 years ago and still does an amazing job whenever I need to shred anything.
- This recipe can be doubled easily, simply choose a larger rectangular casserole dish.
- If you wish to add other root vegetables, add them in their own layer in the middle. Root vegetables can be kept raw, sliced the same thickness as the potatoes, whereas other vegetables such as fennel or onions should be cooked first.
