Turn leftover mashed potatoes into a brilliant cheese pie

By Kate Krader / Bloomberg News

This holiday season, let’s gives thanks for mashed potatoes. They are the MVPs of the industry, a mainstay of weekday dinners, and the anchor of so many festive dinners. According to Google Trends data, they have been, unsurprisingly, the most popular side dish for American Thanksgiving.

Mashed potatoes are, however, invariably a supporting player behind more muscular turkeys, prime ribs, roast chicken, and anything that might come with a bowl of gravy. Jamie Oliver invites you to rethink the possibilities.

The ever-popular British cook catapulted to international prominence as the Naked Chef in the late ’90s. Now, he might be known as the Data Chef. In his new cookbook, “7 Ways: Easy Ideas for Every Day of the Week” (Flatiron Books; $35), Oliver worked with major supermarkets across the U.K. to determine the ingredients consumers buy most.

“My dream was to try and write the most accessible cookbook I’ve ever written,” said Oliver via a phone interview. What that means, he added, is considering what people have on hand, as well as cost, variety and, of course, flavor. “I used data to drive this creation. It’s not glamorous, and I don’t typically like it, but it’s fascinating. We were churning through thousands of items at major supermarkets. I wanted to study food that most of us buy every week.”

From his research, Oliver determined 30 of the most popular foods purchased at supermarkets. He then selected 18 to feature in his book — seven different ways, per the title — with the goal of staving off boredom for home cooks whose supermarket habits are on repeat. They’re all obvious and accessible: salmon, chicken breasts, avocado, shrimp. His steak chapter, for instance, includes South American-styled chimichurri, a Chinese stir-fry with ginger, and a pepper-crusted number with whisky sauce.

Likewise, Oliver’s potato chapter ranges from Indian-accented baked potatoes to a potato “lasagna” (aka gratin) made by layering slices in a creamy cheese sauce. Most compelling is his savory pie with a mashed potato “crust.”

“I pair spring onions and tomatoes with oozy cheese to create the perfect flavor contrast with the soft, creamy mash-which also has a pleasingly crispy outer edge,” Oliver said. In fact, the dish radiates comfort. Oliver calls for two kinds of cheddar in his dish, but it’s wide open to interpretation. “Feel free to mix up the cheese, based on what you’ve got.”

The most important part is the golden potato crust. While mashed potatoes feature as a topping for such classics as shepherd’s pie and moussaka, it’s not so common to find them working as the bottom layer, too. Though it’s firm enough to cut into slices, the pie could double as a dressed-up casserole. You could just spoon it onto plates in one gorgeous pile. The tomatoes add a refreshing bite but they’re not necessary. Hot sauce makes a great addition, if you’re so inclined.

Even as everyone was scaling down this year’s Thanksgiving feast, it’s hard not to overestimate how much mashed potatoes you’ll want. Leftovers are inevitable — and now, very welcome. “This one is perfect if you’ve made a bumper batch of mash,” said Oliver. “Happy holidays!”

The following recipe is adapted from “7 Ways: Easy Ideas for Every Day of the Week,” by Jamie Oliver.

Note: This recipe works best with drier mashed potatoes, ones made without excessive milk and butter. Of course, that’s not what most people want for Thanksgiving. If you have extra creamy mashed potatoes, cook them down so they’re as dense as possible. Mash in an additional boiled potato or two, if you have them. If you have less mashed potato, you can make this pie in a smaller baking dish, with less filling. It’s supremely adaptable.

Crispy cheesy mashed potato pie

3 pounds Yukon gold potatoes; if using leftover mashed potatoes, you’ll need about 6 cups

⅔ pound ripe tomatoes (optional)

1 large bunch scallions, sliced

3 ounces sharp cheddar cheese, coarsely grated

3 ounces aged cheddar, coarsely grated

Salt and freshly ground pepper

4 tablespoons unsalted butter, or to taste

Extra virgin olive oil, for brushing

Hot sauce, for serving (optional)

Peel the potatoes and chop into even-sized chunks. Cook in a large pan of boiling salted water until tender, about 15 minutes. Meanwhile, if using tomatoes, halve them, squeeze out the seeds, and dice the flesh. Toss with a good pinch of sea salt and transfer to a sieve to drain. Press out any excess liquid from the tomatoes and mix with the scallions and cheese. Check for seasoning.

Preheat the oven to 400 degrees. Drain the potatoes, then mash really well with the butter and season to perfection.

Once cool enough to handle, use oiled hands to flatten a little more than half the mashed potatoes in the bottom and a bit up the sides of a 12- by 8-inch nonstick baking pan. Spread the tomato-cheese mixture over the bottom, almost to the edges. Press the remaining mashed potatoes on top, smoothing it out and covering the filling. Make a pattern, if you like on the edge. Brush the top of the pie with olive oil and bake for about 40 minutes, or until golden and crisp. Let cool slightly, then slice. Serve with hot sauce, if desired.

Makes 4 to 6 servings.