Chicken and mushroom gratin

Place chicken in a large saucepan and cover with stock. Bring to the boil, then reduce heat, cover and simmer for 40 minutes. Remove chicken and reserve stock. Discard chicken skin, remove all the meat from the bones and cut into bite-sized pieces.
Heat butter in a saucepan. Add leeks and fry for 1 minute. Add mushrooms and fry until softened. Stir in the flour, then add the milk gradually. Add 200 ml chicken stock from the pot and continue to stir until thickened.
Add chicken meat and sherry and stir well. Transfer mixture to a greased ovenproof dish. Mix breadcrumbs, parsley, Parmesan and lemon rind together and sprinkle over mixture. Bake in preheated oven at 200 ºC for 20 minutes or until golden. Serves 6.