Root Vegetable Gratin

Why this recipe works:

Our root vegetable gratin pairs the earthy flavor of potatoes with thinly sliced sweet rutabaga and savory celery root. We allow these flavors to come to the fore by adding just a few aromatics: onion, garlic, thyme, and Dijon mustard. We add white wine to the gratin because the wine’s acidity strengthens the pectin in the potatoes so that they remain intact while the denser, less-starchy rutabaga and celery root cook through; the wine also brightens the flavor of the vegetables. To help the top layers of the gratin cook through at the same rate, we cover the dish for the first portion of the cooking time. A layer of Parmesan-enhanced panko bread crumbs added after the foil is removed toasts while the gratin finishes cooking, adding a crispy, golden crust that is nutty and cheesy in flavor.

Root Vegetable Gratin

Root vegetables add complex flavor to the all-potato classic, but how do you get the different types to cook evenly? A glass of wine helps. Watch the Video

Serves 6 to 8

Uniformly thin slices are necessary for a cohesive gratin. We recommend a mandoline for quick and even slicing, but a sharp chef’s knife will also work. Because the vegetables in the gratin are tightly packed into the casserole dish, it will still be plenty hot after a 25-minute rest.

Ingredients

• 1 tablespoon plus 1 1/2 cups water
• 1 1/2 teaspoons Dijon mustard
• 2 teaspoons all-purpose flour
• Salt and pepper
• 2/3 cup dry white wine
• 1/2 cup heavy cream
• 1/2 onion, chopped fine
• 1 1/4 teaspoons minced fresh thyme
• 1 garlic clove, minced
• 2 pounds Yukon Gold potatoes, peeled and sliced lengthwise 1/8 inch thick
• 1 large celery root (1 pound), peeled, quartered, and sliced 1/8 inch thick
• 1 pound rutabaga, peeled, quartered, and sliced 1/8 inch thick
• 3/4 cup panko bread crumbs
• 1 1/2 ounces Parmesan cheese, grated (3/4 cup)
• 4 tablespoons unsalted butter, melted and cooled

Instructions

1. 
   
   1. Adjust oven rack to middle position and heat oven to 375 degrees. Grease 13 by 9-inch baking dish. Whisk 1 tablespoon water, mustard, flour, and 1 1/2 teaspoons salt in medium bowl until smooth. Add wine, cream, and remaining 1 1/2 cups water; whisk to combine. Combine onion, thyme, garlic, and 1/4 teaspoon pepper in second bowl.
   2. Layer half of potatoes in prepared dish, arranging so they form even thickness. Sprinkle half of onion mixture evenly over potatoes. Arrange celery root and rutabaga slices in even layer over onions. Sprinkle remaining onion mixture over celery root and rutabaga. Layer remaining potatoes over onions. Slowly pour water mixture over vegetables. Using rubber spatula, gently press down on vegetables to create even, compact layer. Cover tightly with aluminum foil and bake for 50 minutes. Remove foil and continue to bake until knife inserted into center of gratin meets no resistance, 20 to 25 minutes longer.
   3. While gratin bakes, combine panko, Parmesan, and butter in bowl and season with salt and pepper to taste. Remove gratin from oven and sprinkle evenly with panko mixture. Continue to bake until panko is golden brown, 15 to 20 minutes longer. Remove gratin from oven and let stand for 25 minutes. Serve.
   
   
   

Technique

Firming Up Potatoes with Wine

The wine in our root vegetable gratin brightens the flavor of this typically starchy-tasting dish, but more important, it also prevents the potatoes from breaking down and turning mushy while the denser celery root and rutabaga cook. Here’s how it works: Potato cells have an abundance of starch granules; when these granules swell with water during cooking, they press against the cell walls, eventually causing them to burst and release starch. But when potatoes cook in water with wine, the wine lowers the pH, which strengthens the pectin in and around the cell walls, helping them resist bursting. The upshot is a pliable, not mushy, potato.
Of course, the acid also strengthens the pectin in the celery root and rutabaga, but since they contain less starch than potatoes, the acid’s effect on them is less noticeable.
To demonstrate acid’s effect on potatoes, we simmered two batches of 1/8-inch-thick potato slices, one in plain water and the other in 1 1/2 cups of water acidulated with 2/3 cup of white wine. After just 10 minutes of simmering, the potatoes cooked in plain water fell apart, while those cooked in acidulated water remained whole and firm but pliable.

PLIABLE, NOT MUSHY: Cooking potatoes in water with wine strengthens their cell walls, so they resist bursting as they cook.

Technique

Recipe Testing: Four that Failed

In our quest for the best gratin, we tried recipes featuring a range of vegetable preparations: shredded, sliced, cubed, and mashed. But not one lived up to our idea of what a good gratin should be—either because the vegetables were unevenly cooked, the sauce was sticky or soupy, or the flavor was dull and heavy.

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Duck Fat-Roasted Potatoes

Why this recipe works:

Our crispy roasted potatoes are made with Yukon Golds, which give us the best balance of creamy interior and crisp exterior. Duck fat provides unmatched flavor and richness. We peel them for maximum surface area and roast them on a sheet pan on the top shelf of a hot oven to cook them from all sides at once. We boil them briefly in a mild baking soda bath, which degrades the pectin in the spuds’ exterior, and then rough them up in the pot to create a starchy paste that transforms into a crispy shell in the heat of the oven. And we use an easy but effective scrape-and-toss approach rather than turn them one at a time. 

Duck Fat-Roasted Potatoes

For the crispest, most flavorful roasted potatoes, rethink the fat. Watch the Video

Serves 6

Duck fat is available in the meat department in many supermarkets. Alternatively, substitute chicken fat, lard, or a mixture of 3 tablespoons of bacon fat and 3 tablespoons of extra-virgin olive oil.

Ingredients

• 3 1/2 pounds Yukon Gold potatoes, peeled and cut into 1 1/2-inch pieces
• Kosher salt and pepper
• 1/2 teaspoon baking soda
• 6 tablespoons duck fat
• 1 tablespoon chopped fresh rosemary

Instructions

1. 
   
   1. Adjust oven rack to top position, place rimmed baking sheet on rack, and heat oven to 475 degrees.
   2. Bring 10 cups water to boil in Dutch oven over high heat. Add potatoes, 1/3 cup salt, and baking soda. Return to boil and cook for 1 minute. Drain potatoes. Return potatoes to pot and place over low heat. Cook, shaking pot occasionally, until surface moisture has evaporated, about 2 minutes. Remove from heat. Add 5 tablespoons fat and 1 teaspoon salt; mix with rubber spatula until potatoes are coated with thick paste, about 30 seconds.
   3. Remove sheet from oven, transfer potatoes to sheet, and spread into even layer. Roast for 15 minutes.
   4. Remove sheet from oven. Using thin, sharp, metal spatula, turn potatoes. Roast until golden brown, 12 to 15 minutes. While potatoes roast, combine rosemary and remaining 1 tablespoon fat in bowl.
   5. Remove sheet from oven. Spoon rosemary-fat mixture over potatoes and turn again. Continue to roast until potatoes are well browned and rosemary is fragrant, 3 to 5 minutes. Season with salt and pepper to taste. Serve immediately.
   
   

Technique

Saving Fat for Cooking

Saving the leftover fat from bacon, roasts, or poultry is easy to do. Pour the warm fat through a fine-mesh strainer into a Mason jar and discard any solids. (Alternatively, let the fat cool and harden and then scoop it into the jar, leaving any solids behind.) Put the lid on the jar, label it, and refrigerate it for up to one month or freeze it for up to six months, adding more fat as desired.

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