Tarte tatin is a French dessert usually made with apples; the fruit is arranged in a pan of simple caramel and baked with a round of pastry draped over the top. Once the pastry is golden brown and crisp, the tart is inverted onto a plate to reveal shiny, tender, caramel-coated apples.
This version is a savory take, using thinly sliced sweet potatoes (between ¼" and ⅛"—no need to break out a mandoline) and frozen puff pastry. In lieu of making a caramel, you’ll butter and sugar the bottom of the pan, which will liquify and turn caramelly all on its own in the oven. You’ll also brush each layer of sweet potato slices with a spiced harissa mixture, which adds a little heat and earthiness to every bite. A looser and milder harissa, like Mina brand, easily slicks over the potatoes and balances their sweetness well. The final dish makes a quick and easy dinner party side or weeknight vegetarian main—and goes especially nice with a few quick-pickled red onion slices scattered on top.
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Place a rack in middle of oven; preheat to 400°. Unfold 1 sheet frozen puff pastry (one 14-oz. package or half of a 17.3-oz. package), thawed, on a lightly floured surface and roll out to ⅛"–¼" thick. Using a 12"-diameter ovenproof stainless-steel skillet turned upside down as a guide, cut pastry into a 12" round; discard scraps.
Turn skillet right side up and coat bottom and sides with 3 Tbsp. unsalted butter, room temperature. Sprinkle pan with 2 Tbsp. brown sugar in an even layer; season with kosher salt.
Stir ⅓ cup harissa and 1 tsp. ground cumin in a small bowl to combine. Arrange a layer of 2 medium sweet potatoes (about 1 lb.), peeled, sliced into ¼"-thick rounds, slightly overlapping, in skillet; brush with harissa mixture. Repeat with remaining sweet potatoes and harissa mixture to make 2 more layers. Drape pastry over sweet potatoes and tuck edges in between skillet and sweet potatoes. Using a paring knife, cut about six 1" slits all over pastry to let steam vent while baking.
Place skillet over medium-high heat and cook just until sugar and butter start to melt around edges, about 1 minute. Transfer skillet to oven and bake until pastry is golden brown and crisp, 30–35 minutes. Immediately invert tart onto a large plate. (If some sweet potatoes remain stuck to the pan, just peel them off and place them back on the tart; no one will be any the wiser.)
While the tart is baking, place 1 small red onion, thinly sliced, in a small heatproof bowl. Bring ½ cup white wine vinegar, 1 Tbsp. coriander seeds, and remaining 2 tsp. brown sugar to a boil in a small saucepan, then pour over red onion. Season generously with kosher salt and stir to combine. Let cool.
Sprinkle tarte Tatin with flaky sea salt and cut into wedges. Serve with drained pickled red onion.