In a large bowl, combine 250g plain flour and 1 teaspoon salt. Rub in lard until mixture resembles coarse crumbs. Stir in water until mixture forms a ball. Divide dough in half and shape into 2 balls. Wrap in cling film and refrigerate for 4 hours or overnight. Roll one ball out to fit a 23cm pie dish; place in pie dish. Roll out the other ball to make the top layer of pastry and set aside.
Preheat oven to 200 C / Gas 6.
In a large saucepan, melt 115g butter. Blend in 60g flour, salt, black pepper and chopped onion. Gradually stir in chicken stock. Cook, stirring constantly until smooth and thickened. In a separate pan, cook mushrooms in 3 tablespoons of butter until softened; add mushrooms back into saucepan. Add chicken, carrot, celery and potatoes. Mix well and pour onto the pastry in the pie dish. Cover with top layer of pastry; seal edges and cut away any excess pastry. Make several small slits in the top to allow steam to escape.
Bake in the preheated oven for 30 minutes, or until pastry is golden brown, and filling is bubbly.