3 tablespoons unsalted butter
1 tablespoon sugar
8 small sweet onions, peeled, ends removed and halved crosswise
1 teaspoon kosher salt
2 tablespoons balsamic vinegar
1 frozen puff pastry sheet, thawed
Fresh thyme sprigs and freshly cracked black pepper, for garnish
Preheat oven to 375 F. Melt butter in a 12-inch ovenproof cast-iron skillet over medium-low heat; swirl to coat bottom. Sprinkle the bottom of the pan evenly with sugar.
Place 12 to 14 onion halves in the skillet, cut sides down, with the sides touching. Cut remaining onion halves into quarters, and place, cut sides down, in gaps between onions in skillet.
Cover and cook onions 10 minutes, adding salt and 1 tablespoon of water halfway through. Add vinegar; cover and cook 3 to 5 minutes or until onions are caramelized. Remove from heat.
Roll pastry sheet out on a lightly floured work surface; cut a round the same diameter as the skillet. Place pastry round over onions in skillet.
Bake for 30 minutes or until golden. Remove from oven; immediately invert onto a serving plate. Garnish with thyme and freshly cracked pepper. Slice and serve warm.