Remove the lemon rind using a swivel peeler, squeeze the juice and reserve. Tuck the lemon rind inside the chicken, then put it in a large pan with the carrot, celery, onion and peppercorns. Add enough cold water to cover the chicken thighs and cover the pan with a lid. Bring to the boil, then simmer for 45 minutes, or until the juices run clear when the thigh is pierced. Lift the chicken from the pan and put to one side to cool. Bring the stock to the boil and bubble until about 600 ml (1 pint) is left, then strain and reserve the liquid.

Preheat the oven to 200°C/180°C fan oven/Gas Mark 6. In a frying pan, melt 50 g (2 oz) of the butter and fry the leeks for 4 to 5 minutes or until lightly coloured. Stir in the flour and cook, stirring, for 2 to 3 minutes, then add the reserved chicken stock and milk. Bring to the boil, stirring all the time. Simmer for 2 to 3 minutes, or until the sauce thickens. Season, add the remaining lemon juice and allow to cool.

Remove the skin from the chicken and cut the flesh into bite-sized pieces. Stir the chicken into the sauce, then spoon this into a 2.6 to 3 litre (4 1/2 - 5 1/4 pint) ovenproof dish.

Melt the remaining butter. Crumble the bread and mix with the butter, adding the garlic and walnuts. Sprinkle over the chicken. Cook for 20 minutes, then cover with foil and cook for 10 to 20 minutes or until golden. Garnish with oregano sprigs.

Freezing: Complete the recipe but don't cook the pie. Wrap and freeze.

To use: Thaw overnight at cool room temperature. Add an extra 5 - 10 minutes to the cooking time and cover the top with foil if the pie begins to turn too brown.