Unroll the pastry sheet. Cut the camembert wheel in half, and put it in the center of the pastry.
Put the prosciutto slice on it, then the minced and caramelized onions.
Close the camembert with its other half, and cut the pastry around it in 8. Fold it back on the camembert. Brush with the beaten egg.
Bake 20 minutes at 375°F (190°C).
There you are, your flaky camembert is ready!