1 teaspoon
fresh thyme leaves, plus more for serving
1 sheet
frozen puff pastry, thawed
Flour, for dusting
4 ounces
cream cheese, room temperature
1 teaspoon
Dijon mustard
6 ounces
Swiss cheese, shredded
Balsamic glaze, for drizzling
Freshly cracked black pepper, to taste (optional)
Flaky salt, to taste (optional)
Method
Melt the butter in a large nonstick skillet over medium heat. Once the butter has melted, add the onions and cook, stirring often, until softened and translucent, about 10 minutes. Reduce the heat to medium-low, then spread the onions into a single layer and cook, stirring and returning to an even layer every 10 to 15 minutes, until the onions are golden brown and caramelized, about 45 minutes total. If the onions are browning too quickly, add water 1 tablespoon at a time until they return to caramelizing. Taste and season with salt, then stir in the brown sugar and thyme.
Preheat the oven to 375°F. Line a sheet pan with parchment paper.
Unroll the puff pastry on a lightly floured surface, then roll it out into an 11 x 14-inch rectangle.
In a small bowl, combine the cream cheese and mustard until smooth. Spread a thin layer of the cream cheese mixture over the puff pastry leaving a 1-inch border. Top with an even layer of the caramelized onion, then sprinkle with the Swiss cheese. Tightly roll the dough over the filling, using a little water to seal the seam. Slice the roll into ½-inch-thick slices and arrange them on the prepared sheet. Bake until lightly browned and puffed up, 15 to 20 minutes.
Before serving, lightly drizzle with the balsamic glaze and garnish with thyme. Sprinkle with pepper and flaky salt, if desired.