Preheat oven to 350 °. Lightly grease a cookie sheet.
Slice the wheel of Brie in half, horizontally, so that the halves are two, flatter wheels of Brie.
Lay the puff pastry on the cookie sheet. Place half of the Brie, rind-side down, onto the pastry dough.
Spoon caramelized onions evenly over the top. Place the other half of Brie, rind-side up, over the caramelized onions.
Fold the puff pastry up and over the Brie, sealing so it is bundled up. Flip entire Brie over so seal is on the bottom.
Bake for 20-25 minutes, or until puff pastry is golden brown. Server warm with crackers or baked pita chips.