Found: A truly excellent potato side dish. The fact that it's portable enough to carry off to holiday dinners elsewhere adds to its charm. And I found that it's flexible - once you learn the basic recipe, the theory will let the cook make it larger or smaller as the number of diners vary.
Credit for this recipe goes to the fine website Food 52, who had it entered into a contest from the Canadian website Katheryn's Kitchen.What initially attracted me to it is the idea of caramelized onions, always a superb partner to the potato's charms. I'd never had a potato gratin recipe that totally satisfied me until this, and as I researched the best way to carry this recipe off, I learned several things.
One is that the thinner the potatoes are sliced, the better. They not only cook more quickly, the more potato surface is exposed, allowing for more potato starch to thicken the cooking liquid. Shallow is better than deep, allowing for more crusty top and also making for more even coking. This is a dish that calls for low and slow. Not to worry if you can't manage the time for this before dinner - it reheats very well, covered with foil at 375 degrees for about 30 minutes; just be sure you have a baking dish that can handle that.
You can slice the potatoes and onions in a food processor, although I didn't. My knife skills are that of an experienced amateur - not all the potato and onion slices were even. I tried to put any noticeably thicker slices at the corners and sides of the baking dish, thinking they would get more heat there than in the center. Yes, whipping cream - I haven't tried it with lower fat dairy products, like using whole milk for part of the liquid. You're on your own there. Yukon Gold potatoes are the way to go here, by the way, softening nicely but keeping their shape.
The original recipe called for a dish "about 8 x 12 inches". I used an oval gratin about 9 x 12 inches when I made this the first time. The second time, I made a batch half again larger - 3 cups of cream, for instance - and used a 9 x 13 glass baking dish.
Interestingly, the onions seem to dissolve almost completely, adding to the mystery. Letting it stand after it's removed from the oven is helpful, allowing it to set up and become a little more solid.
2 c. whipping cream
4 garlic cloves crushed
3/4 tsp. salt
1/2 tsp. dried thyme
1 Tbs. oil plus more for greasing pan
1 large white or yellow onion, thinly sliced
2-3 lbs. Yukon Gold potatoes
1 c. shredded Gruyere cheese
In a saucepan, combine the cream, garlic, peppercorns, salt and thyme. Bring almost to a boil over high heat, drop the heat to very low, cover the pan and cook for 15 minutes. Remove from the heat.
Meanwhile, put a skillet over medium-high heat. When hot, add the oil and swirl around to cover the bottom. Add the onions. Saute, stirring occasionally, until golden brown. This may take 30-45 minutes.
Preheat oven to 300 degrees and grease baking dish. Thinly slice potatoes and lay out a single layer in the bottom of the dish. Sprinkle about a third of the onions over the potatoes and then a fourth of the cheese. Add another layer of potatoes, another third of the onions and another fourth of the cheese, then repeat again for a third layer. Finish up with a last layer of potatoes.
Using a sieve, strain the cream onto the potatoes. It won't cover them completely, but shake the pan gently to help eliminate any air bubbles. Sprinkle with the last of the cheese.
You may want to place the baking dish on another pan in case it bubbles over - mine didn't. Bake 90 minutes - 2 hours. It should be bubbling all over, and golden. A sharp knife inserted into the center of the pan should encounter no resistance.
Serves 6-8 - but as I always say, one never knows.