1 (14-ounce) sheet puff pastry, defrosted in refrigerator overnight
2 tablespoons extra-virgin olive oil
1 leek, white and light-green parts, sliced thin
10 ounces Brussels sprouts, halved
Salt, to taste
Ground black pepper, to taste
2 tablespoons balsamic vinegar
1 tablespoon finely chopped, fresh thyme leaves
4 ounces shredded Gruyere cheese
1 large egg, lightly beaten
Line a baking sheet with parchment paper. Unfold the puff pastry and place in middle of baking sheet. Prick all over with a fork. Cover with plastic and place in freezer.
Preheat oven to 425 F. Heat the olive oil in a large, nonstick skillet over medium-high heat. Add the leek and cook until softened, about 3 minutes. Add the Brussels sprouts, salt and pepper to taste and cook, stirring occasionally, until sprouts are spotty brown, 5 to 7 minutes. Add the balsamic vinegar and continue to cook, stirring, until most liquid has cooked off (this will take less than 30 seconds). Scrape into a bowl, stir in the thyme and set aside to cool.
Remove puff pastry from freezer and spread cooled sprouts mixture over it, leaving a 1-inch border all around. Sprinkle the cheese over Brussels sprouts. Brush edge of dough with the egg. Bake in preheated oven until pastry is golden and cheese is browned and bubbling, about 20 minutes. Cool slightly and serve warm or let come to room temperature before serving.
Makes 4 to 6 servings.