Cut chicken from bones. Remove skin and discard.
Melt butter and fry onions for 5 minutes or until golden. Stir in flour gradually and cook for 1 min. Remove from heat and gradually stir in stock and cider. Bring to boil while stirring, while adding in carrots, peas, and sweetcorn. Season with salt and pepper, then leave to cool.
Divide pastry into 2 amounts, one 1/3 and the other 2/3. Roll out the larger one on floured surface to form the base. Place in a 12” pie dish. Spoon mixture into pie dish.
Roll out leftover pastry to form the lid. Place over the pie. Trim excess pastry and moisten the edges of the base pastry with milk to form a seal. Brush top with milk and use the trimmings to decorate.