Heat the oven to 200C/fan 180C/gas 6. Roll the block of pastry to a 50 x 30cm rectangle and cut into 15 10 x 10cm squares. Brush a little mustard into the middle of one square and put 1½ tbsp of mince and stuffing mixture in the middle. Make an indent in the stuffing and put 1 tsp of cranberry sauce. Fold the four corners into the middle, making a loose parcel, and seal lightly at the top. Transfer to a baking-paper-lined tray, brush with beaten egg and sprinkle with onion seeds. Repeat with the remaining dough and filling. At this stage you can freeze these on the tray and, once frozen, transfer to freezer bags. To serve, defrost in the fridge overnight.