Using a large bowl, cut out 2 circles from the puff pastry. Transfer them to a tray, wrap them in parchment paper and put them in the freezer to firm up while preparing the filling;
Finely chop (using a food processor) the onion and the ham separately and set aside;
Sauté the onion in butter over medium-low heat until golden. Let it cool;
In a large bowl, combine the ham and onion, then gradually mix in the cream, which is your filling;
Remove one of puff pastry circles from the freezer. Place it on a tray, covered with parchment paper, and brush around the edge with egg about 2 cm from the periphery;
Spread the filling in an even flat layer inwards from the egg;
Sprinkle grated cheese on top and place the second circle of puff pastry on top;
Seal the edges by gently pressing together with your palm side;
Place a cup in the center and carefully press to mark a circle without tearing the dough;
Then cut the dough crosswise, starting from the circle, first into 4 quarters and cut each of them in half again;
Grab the outer end of each slice and gently twist it to create a 'sunbeam'. Repeat this with each of the slices;
Brush the top with an egg wash and put it in the fridge to chill for 15 minutes;
While the tart is cooling, preheat the oven to 180℃ with fan;
Bake for 30 minutes until puffed and golden;
Serve so that everyone can reach and cut off a "sunbeam".