Peel the onion and chop it finely. Saute it in the oil and add the mince. Braise 4-5 min. Let the stuffing cool.
Lay out the puff pastry and cut out circles using cookie cutters. In half of them, cut out holes using a smaller circle. Put 1 tbsp of filling onto each circle without a hole.
Place a circle with a hole on top and press the edges together using a fork. Put a piece of cheese on top of the mince in each circle.
Smear with beaten egg yolk on top. Do the same for the rest of the circles. Arrange the puff pastries in an oven dish with baking paper. Bake in a preheated oven until golden.