Sweet potato-Gruyere gratin

While winter in Southern California is short on blizzards and subzero temperatures, it does have plenty of chilly winds and rainy days. When they hit, there’s only one thing to do: Grab a chair by the fire and pick up a good book -- preferably one with lots of gratins in it.

Basically nothing more than vegetables and cream baked together until they fuse into one, gratins couldn’t be easier to make, which is certainly not to say that they lack complexity. The vegetable soaks up the liquid to the point that it becomes meltingly tender. The cream takes the essence of the vegetable and enriches it. And the crusty layer on top combines the best of both and gilds it a lovely brown, adding a slight crispness and a deeply satisfying flavor.

No wonder the dish is so beloved. And while it seems impossible that something so simple and so delicious could be an object of controversy, you know how cookbook writers can be. When it comes to gratins, nobody agrees on anything.

Edouard de Pomiane, one of the great mid-century French culinary writers, was speaking of gratins when he wrote, “There are certain dishes the very name of which always arouses a storm.”

In “Cooking With Pomiane,” first published in the 1930s, he describes being served a gratin dauphinois -- the archetypal potato gratin -- at a dinner of the Academie des Gastronomes. The dish was a flop. “Gravely, three members of the Academie des Gastronomes rose to their feet and gave their opinion on the way a gratin should be made.” Though they all agreed that the gratin served was horrible, predictably, they differed on how it should have been done.

De Pomiane’s version is a model of simplicity. Slice 1 1/2 pounds of potatoes very thin. Mince four cloves of garlic. Cover the bottom of a baking dish with half of the potatoes. Sprinkle with salt, pepper and half of the garlic. Top with the remaining potatoes and season again. Bring 1 1/2 cups of milk to a simmer and pour it over the potatoes. Mix two-thirds cup of whipping cream with one-half teaspoon of flour and pour that over the potatoes. Bake at 400 degrees until brown, about an hour.

Roy Andries de Groot disagreed. In his cult favorite book “The Auberge of the Flowering Hearth,” which romantically recounts his stay at the perfect small hotel in the French Alps, his gratin differs from De Pomiane’s on almost every important count. First, he precooks the sliced potatoes in the milk on top of the stove. Then he bakes them at a much lower temperature (300 degrees) in a covered casserole set in a water bath.

Madeleine Kamman, whose family has roots in the same area, devotes an entire chapter to gratins in her book on the Savoie. She seconds De Groot’s low-heat technique but rejects everything else. Her version in the new edition of “The Making of a Cook” is perfect in its way: Rub a gratin dish with garlic and butter it well. Slice 1 1/2 pounds of potatoes very thin. Toss them with salt, pepper and nutmeg and arrange them in the pan. Cover with 1 1/2 cups of whipping cream and bake at 325 degrees until brown, about 1 1/2 hours (she also provides the great tip that you should periodically break the crust that forms and submerge it in the unbrowned cream).

The noted American expatriate author Richard Olney comes down in De Pomiane’s camp in almost every respect. Indeed, his recipe in “Simple French Food” is virtually identical, with the exception that he brings the potatoes and milk to a simmer together in a baking dish on top of the stove before sticking it in the oven.

As with anything so simple, variations are legion.

Poach the potatoes in stock instead of milk before adding the cream and your gratin is a “Savoyard.” That’s just the start. Kamman includes half a dozen twists on the basic potato gratin in “The Making of a Cook,” including adding mushrooms, blue cheese, minced onions, leeks, celeriac, fennel and mustard (separately, not together).

One of my favorite gratins is made with potatoes and turnips, kind of a glorified version of the Scottish “Neeps and Tatties.” Bake two sliced turnips and six sliced potatoes in a covered gratin dish in a 450-degree oven until they begin to soften. Cover them with 1 1/2 cups of cream, scatter over a couple of ounces of grated Gruyere and bake until brown.

Some advise specific baking dishes (De Pomiane requires round: “For some reason an oblong dish is not considered quite right”). Some advise that you must rinse the potato slices thoroughly and then pat them dry to rid them of extra moisture. Others say just to pat them dry. Finally, there are those who ignore that step altogether.

Some authors specify boiling potatoes, others baking. Theoretically, this should be an important distinction, but I tested them side by side and have to confess that I couldn’t find a difference.

But gratins don’t even need to contain potatoes at all. The charm of the dish lies as much in the deep flavor of cooked cream as it does in the comforting heft of starch. Actually, the name itself, derived from the French verb gratter (the same root as the English “to grate”), refers to the browned bits of butter crust that appear on top.

Indeed, after a couple of weeks of cooking gratins of various stripes, I’m thinking about trying out a new one: the air gratin, made with nothing but heavy cream slowly baked until it is brown and delicious.