Gently sauté the onions in the oil in a large pan, then add the chicken and cook, stirring, until all sides browned or lightly coloured.
Stir in contents of the cans of tomatoes (one can be drained, use flesh only), potato and seasonings, bring to boil then gently simmer, stirring occasionally, for 18-20 minutes until sauce thickened. Allow mixture to cool.
To assemble parcel, take one thawed pastry square and place 60g cooled filling in centre. Brush edges with beaten egg then bring in the four corners to meet over the filling. Press edges together to seal and gently knock up along seams. Don’t worry if the centre is not entirely sealed as the finished parcel will open slightly to reveal the filling.
Brush tops with beaten egg and continue with all remaining ingredients. Bake for approx. 15 minutes until pastry risen and golden brown. Serve hot with vegetables or salad.