Grease a tart pan or muffin molds with butter, dust with flour and set aside. Peel the onions and cut lengthways into 5 mm thick rings. Leave the ham cubes crispy in a pan, add the onion rings and sweat for 3 minutes while stirring.
Whisk the sour cream with the eggs and season well with salt, colored pepper and oregano. Cut the apples into wedges and drizzle with lemon juice.
Either place the pizza dough in the tart pan and cut off the protruding edge up to 2 cm or cut out circles and place them in muffin tins. Pour the onion bacon mixture on the dough, pour the egg cream over it and spread the apple wedges on the tartlets.
On the large tart, fold the protruding edge inwards in a wave shape and brush with whisked egg yolk. Bake in the oven at 180 ° C for 40 minutes (tart) or 20 minutes (muffins).
Tip: stir finely chopped chives and parsley into the egg cream. Garnish the tart with parsley and chives.