Leek Upside-down Tart

Seasonal Recipe of the Pandemic
Here is a very cool and unusual recipe I found online and adapted to make it even better. It's very easy but looks very complex. If you follow the directions exactly it will be JLP (just like the picture). A great one to bring to a party if you want to impress. Or make one for yourself and have it with some prosciutto and a nice glass of wine.

  

 

Ingredients
1 14-ounce package frozen puff pastry, thawed
6 leeks
2 tablespoons extra-virgin olive oil
1/2 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper, plus more
1/4 cup aged sherry vinegar
2 tablespoons unsalted butter
1 teaspoon sugar
1 teaspoon thyme leaves, plus more for serving
2 tablespoons Dijon mustard
2 ounces Asiago cheese, grated (about 1/2 cup)
1 tablespoon shredded Parmesan cheese
flaky sea salt and black pepper

Procedure
Place a rack in center of oven; preheat to 400°. Roll out puff pastry on a lightly floured surface to a 12" square. (If you have 2 sheets of pastry, stack tham and gently roll out to stick them together.) Using a 10" cast-iron skillet turned upside down as your guide, trace a circle with a sharp knife around circumference of skillet. Reserve scraps for another use. Prick pastry round all over with a fork. Roll dough onto rolling pin, then unfurl onto a rimmed baking sheet and chill until ready to use.

Trim the root ends and dark green parts off the leeks. Then cut them in half lengthwise and run under cold water while fanning out the layers to remove any sand. Toss leeks, oil, kosher salt, and 1/2 tsp. pepper on another rimmed baking sheet. Arrange leeks cut sides down and roast until tender and lightly browned in spots, about 20 minutes. Let cool. Remove tough, leathery outer layer, then transfer leeks to a cutting board.

Meanwhile, bring vinegar to a simmer in 10" cast-iron skillet over medium-high heat and cook, stirring occasionally, until slightly reduced, about 1 minute. Add butter, sugar, and 1 teaspoon thyme and continue to cook, stirring occasionally, until butter has melted, about 2 minutes. Remove from heat and let cool slightly.

Arrange leeks cut sides down and side by side in rows in vinegar mixture, trimming as needed so they all fit snugly in skillet. Fill any holes with small pieces. You may have a leek or two left, so just eat them. Brush leeks with mustard. Sprinkle with the Asiago and Parmesan cheeses. Drape chilled pastry over leeks, tucking edges inside skillet with a spoon. Cut three 3-inch slits into pastry (be sure to cut all the way through) to let steam escape—this will help the pastry cook more evenly. Bake tart until pastry is deeply golden brown and puffed, 35–45 minutes.

Let tart cool slightly, about 5 minutes. Carefully place a large plate on top of skillet. Using towels, invert tart onto plate. If some of the leeks get left behind in skillet, use a butter knife or offset spatula to release and nestle them back into tart. Garnish with the extra thyme, sea salt, and pepper then pour me a glass of wine!