Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
Unfold the thawed puff pastry onto the prepared baking sheet. Use a fork to prick the center of the pastry all over, leaving a 1-inch border around the edges.
Arrange the brie slices evenly over the pricked center of the pastry, staying within the border.
Spoon the cranberry sauce over the brie and spread gently with the back of a spoon.
Scatter the diced pancetta evenly over the cranberry layer.
Brush the pastry border with the beaten egg wash.
Bake for 20-25 minutes until the pastry is puffed and golden brown and the cheese is melted and bubbly.
Remove from the oven and immediately sprinkle fresh thyme leaves and garnish with thyme sprigs. Let cool for 2-3 minutes before slicing and serving.