INGREDIENTS
•
3 tablespoons butter
•
1 tablespoon sugar
•
8 small sweet onions (about 1 1/2 lb.), halved crosswise
•
1 teaspoon kosher salt
•
2 tablespoons balsamic vinegar
•
1 frozen puff pastry sheet, thawed
•
Garnishes: fresh thyme sprigs, freshly ground black pepper