Divide the puff pastry sheets into 8 squares each.
Finely dice the ham and cut the onions into fine rings. Mix in a baking bowl with the Parmesan cheese and the sour cream or crème fraiche, on condition that the quantity is too firm, possibly fold in a little whipped cream.
Brush the edges of the puff pastry squares with egg white. Place the ham mixture evenly on one corner of each square. Fold the dough over and roll up into croissants. Place on a baking tray covered with parchment paper and brush with the egg yolk mixed with a little whipped cream.
Bake in a heated oven at 200 °C for about 25 minutes.
Tip: It is best to use your favorite ham – then your dish will taste twice as good!