Thursday, November 18, 2010

Sweet Potato Gratin with Turnips & Yukon Gold Potatoes

In my parade of Thanksgiving recipes, it would be a shame if I didn't include a vegetable gratin. Always well-received, they are beautiful, versatile and very satisfying. I love them.

Technically, any vegetable, or combination of vegetables, that has been baked in a shallow dish in such a way that it has become browned on top—the word gratin is from the French verb gratiner which means "to brown"—can be called a vegetable gratin. But for my holiday purposes, I am referring specifically to a dish in the style of the famed Gratin Dauphinois—thinly sliced potatoes, layered with heavy cream and baked slowly to melting tenderness. By the time the Gratin Dauphinois is finished baking, it has acquired a beautiful burnished brown surface (even if it doesn't have cheese on top). Gratins are always baked in a wide shallow dish so that there will be as much surface area as possible available for browning. In fact, the classic oval dish used for baking them is commonly called a Gratin....

I frequently make the classic potato version, but I also love to make gratins with potatoes in combination with other vegetables....particularly the root vegetables. Almost any root vegetable will be at home in a potato gratin. People who think they don't like turnips or parsnips might be persuaded to try them if they are presented with a bubbling dish of creamy, cheese-topped potatoes that just happens to include a few turnips or parsnips.

For many years, my favorite holiday version of the gratin was one made of Idaho Potatoes and Butternut Squash. I never get tired of this combination and I have taught it several times. But for my Winter Squash and Sweet Potatoes class this year I thought I would teach a gratin with sweet potatoes and use the squash in other preparations.  If you happen to have some family members who want white potatoes for Thanksgiving and others who think that the holiday table isn't complete without sweet potatoes, this gratin might be a good way to please everyone. 

In addition to the sweet potatoes, I included some turnips in the gratin. The slightly bitter turnips make a great partner for the sweet potatoes. This gratin would be a great way to introduce turnips to someone who has never had them. It would also be a good side dish to try if you are tired of the extreme sweetness of the standard sweet potato dishes.

In one respect, this gratin is a bit of a departure for me. I generally make my gratins with all cream. Recently I have been experimenting with incorporating a bit of stock into my root vegetable gratins. My purpose in doing this is to achieve a gratin that isn't so shockingly rich. Holiday meals are rich enough. Even a small portion of a gratin made with all cream can push you over the edge from being a little bit too full to feeling ill. With this gratin, I think I have hit upon a ratio of cream to vegetables that works well. I use a half cup of cream for each pound of root vegetables—this makes for a gratin that is not quite so rich, but is still satisfyingly creamy. I include directions in a footnote to the recipe if you would like to use less cream...or if you would like to use all cream.

If you need to make your gratin ahead, you can build and bake it the day before or early in the day on the day you will be serving it. Remove it from the oven when the vegetables are just tender and before the cream has reduced to the point when it is "bubbling thickly"—perhaps after 45 minutes. If you bake the gratin until the cream is fully reduced (as you would if you were serving it right away), the cream will continue to reduce when the gratin is reheated and it will break into butter and milk solids and will look curdled. It will still taste good, but it will no longer be beautiful and creamy. After removing the gratin from the oven, cool to room temperature and then refrigerate. Before serving, bring the gratin back to room temperature. Cover loosely with foil and bake at 350° until hot through—30 minutes or so.

I should probably mention that if you are comparing my recipe to the gratin in the pictures, you will notice that the one pictured is not a full recipe.  It is just under half of a recipe.  When I began to plan my Sunday dinner this past week I discovered that I had one lonely turnip and one lonely Yukon gold potato left from my last trip of the season to the farmers' market.  I decided to make a small version of this gratin to go with a roast chicken.
The reason that I mention this is that it gives me a good opportunity to point out that although you should feel free to multiply or divide a gratin to suit your needs, you should be aware that the amount of liquid may not multiply or divide proportionately to the vegetables.  For half a recipe of this gratin, you would expect to use 3/4 cup of cream and 1/4 cup of stock.  In practice I used 3/4 c. of cream and somewhere between 1/3 and 1/2 cup of stock.  The amount of liquid needed in a gratin of any size can vary a bit.  As always, it is important not to just blindly follow the recipe.  When adding liquid to a gratin, add just enough so that when the vegetables are pressed , they are only partially submerged:

When you are not pressing down on the vegetables, the liquid will just barely be visible around the edges:

Too much liquid and a gratin will be soupy...not enough and it will be dry and won't cook evenly.  For this particular gratin, follow the rule of thumb of 1/2 cup of cream per pound of vegetables and then add stock until the level of liquid in the pan is as described.

The gratin was very good with our roast chicken--almost a mini Thanksgiving.  And it was an especially nice way to say good-bye to the tail end of my local vegetables for the season.


Sweet Potato Gratin with Turnips & Yukon Gold Potatoes

1 T. butter
1 lb. Sweet Potatoes
1 lb. Turnips
1 lb. Yukon Gold potatoes
1 1/2 c. Heavy Cream
1/2 c. chicken stock or low-sodium canned broth
2 cloves garlic, finely minced
1 T. chopped fresh thyme
Salt & freshly ground pepper
4 oz. coarsely grated Gruyère

Butter a 2 1/2- to 3-quart shallow gratin; set aside.

Peel all of the vegetables and slice very thinly (1/16-inch thick) cross-wise. This is most easily done with a mandoline slicer. Peel and slice the Yukon potatoes last so they won't oxidize.

While you are peeling and slicing the vegetables, bring the cream and stock to a simmer. Remove from the heat and season well with salt and pepper.

Shingle the turnips in and even layer in the baking dish. Scatter half of the thyme and garlic over the turnips and season lightly with salt & pepper.

Moisten with some of the cream/stock mixture. In like manner, layer in the Yukon potatoes, seasoning with half the thyme and garlic, and some salt & pepper. Moisten with some of the cream/stock mixture.

Finish the gratin with an attractive layer of shingled sweet potatoes, seasoning again with salt & pepper.

Add enough of the remaining cream/stock mixture so that the vegetables are just covered when lightly pressed. Scatter the Gruyère evenly over the gratin.

Place the gratin on a baking sheet and bake in a preheated 350° oven until the vegetables are very tender, the cream is reduced and bubbling thickly and the top is golden brown—1 hour to 1 hour and 15 minutes. The gratin may be served immediately or kept in a low oven (less than 200°) for up to 2 hours. Serves 8 to 10.

Note: This is a lighter version of a traditional gratin. A classic gratin is made entirely with heavy cream. If you prefer the richer, classic version, use 2 cups of heavy cream and lower the oven temperature to 325°. The gratin will take 15 to 30 minutes longer to cook. For an even lighter version, use 1 cup stock and 1 cup heavy cream and bake at 375° for about an hour.P

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