Preheat oven to 400°F. Remove the top layer of rind from the wheel of brie. Place the onion jam on open face of the brie. Set aside.
Place puff pastry sheet on a flour dusted surface and roll out so it is roughly 1" longer on each side. Place overtop of the onion jam on the brie and invert the wheel so that the pastry is on the bottom and the brie is facing up.
In a small bowl beat egg with a small amount of water. Fold the sides of the pastry around the brie, brushing with the egg wash to create a seal. When all sides are folding in, flip wheel so the top is smooth pastry and place on a parchment lined sheet tray. Brush with egg wash.
Bake until pastry is golden brown, about 25-30 minutes. Serve with crackers or slices of bagette as a shareable appetizer.