Preheat your oven to 375°F.

In a large skillet over medium heat, melt the butter. Add the sliced onions and cook, stirring often, for about 20–25 minutes until they are soft and golden brown. If desired, add the brown sugar and balsamic vinegar during the last few minutes to enhance flavor. Remove from heat and let cool slightly.

On a lightly floured surface, roll out the puff pastry just a little to smooth out any creases.

Place the Brie round in the center of the puff pastry. Top the Brie with the caramelized onions.

Fold the puff pastry up and over the Brie, sealing all edges at the top. Trim off any excess pastry if needed and press to seal.

Brush the entire pastry with the beaten egg for a golden finish.

Place on a parchment-lined baking sheet and bake for 25–30 minutes, until the pastry is golden brown.

Let it cool for 5–10 minutes before serving. Serve warm with crackers, baguette slices, or apple slices.