For the filling, heat the oil in a frying pan and cook and stir the onions until golden. Stir in the beef mince, garlic, parsley, green pepper and the stock cube, and cook for 10 minutes. Set aside to cool.
Roll out puff pastry and cut into 8 to 12 pieces. Add a small amount of cooled filling in the centre of each and fold over. To seal the edges, brush with beaten egg.
Brush turnovers with beaten egg and bake in the preheated oven until golden, about 30 minutes.