Place the chicken in a pan with water, bay leaf and seasoning. Bring slowly to the boil, cover tightly and simmer very gently until the meat is tender; it will take fifty minutes if using a roasting chicken, or two and a half hours if using a boiling chicken. Remove the chicken meat from the carcass and chop it roughly. Skim the fat from the pan, remove the bay leaf and reserve the stock. Melt the butter in a pan and fry the chopped onion gently for five minutes. Add the chopped mushrooms and cook for a further two minutes. Add the chicken meat and herbs, and season to taste. Cool.
For the pastry, sift together the flour and the salt and rub in the fats until the mixture resembles fine breadcrumbs. Add the water and mix to a firm dough. Turn on to a floured surface, and knead lightly until smooth. Roll out into a circle approximately thirteen inches in diameter. Cut out a section representing quarter of the circle and reserve for the lid. Line a round cake tin seven inches in diameter and three inches deep with the larger piece of pastry, joining cut edges. Spoon half of the chicken mixture into the tin and press down.
Arrange the hard-boiled eggs on top and cover with the remaining chicken mixture, pressing it down between the eggs to fill the spaces. Spoon over four tablespoons of the chicken stock. Damp the edges of the pastry. Roll out the remaining pastry for the lid and cover the pie. Trim off excess pastry, press edges together and flute. Roll out trimmings and cut leaves to decorate centre of pie. Brush with glaze and make a hole in the centre of the pie. Bake at 400°F, 200°C, gas mark 6 for thirty minutes. Reduce heat to 350°F, 180°C, gas mark 4 for a further thirty minutes. Leave to cool in the tin. Remove from the tin and pour stock through the hole in the centre until the pie is full. Leave to set.