4 Tbsp. salted butter, melted
12 medium shallots (about 12 ounces), peeled and halved lengthwise
1/2 cup turbinado sugar
All-purpose flour, for rolling
1 pound puff pastry, thawed and chilled
1/2 cup balsamic vinegar
2 ounces Gorgonzola, crumbled
Sea salt, for sprinkling

Preheat oven to 400° and arrange racks in middle and lower third. Line 2 baking sheets with parchment paper. In a medium bowl, combine melted butter, shallots, and sugar, and toss to coat. Arrange shallots cut-side down on prepared baking sheets. Scrape any remaining sugar mixture onto shallots. On a floured surface, roll puff pastry to a 15″ square. Using a 2 1/2″ round biscuit cutter, stamp out as many circles as possible. If necessary, gently press scraps together and freeze until firm, then reroll and cut out more rounds. Place pastry rounds over shallots, and top each pan with a sheet of parchment and another baking sheet. Bake for 20 minutes, switching position of pans halfway through. Carefully remove top baking sheets and parchment paper. Return tartes tatin to oven and continue to bake until pastry is golden, 15 to 20 minutes more, switching pans again halfway through. Working quickly, using a spatula, invert tartes tatin onto a serving platter. Meanwhile, in a small saucepan, boil balsamic vinegar until reduced to 2 Tbsp., about 5 minutes. Dot tartes tatin with Gorgonzola and drizzle with balsamic reduction. Sprinkle with salt and serve.