Cut the bratwurst into small pieces. Finely chop the onion and mix with the sour cream and quark. Stir in the bratwurst pieces.
Thaw the puff pastry for about 10 minutes. Roll out lightly and cut into squares. Place the filling in the middle of each square and fold the pastry over to enclose. Press the edges well to seal. Bake in a preheated oven at 200°C (392°F) for 15 minutes until golden brown.