Soak the sago in 250 ml cold water for 30 minutes.
Bruise the coriander, cloves and peppercorns in a pestle and mortar or with a hammer.
Place chickens, bay leaves, onion, spices and chicken stock in a large pot. Simmer until the chicken is just cooked (about 45 minutes).
Remove the chickens from the pot and leave to cool. Skin, debone and dice the chickens into 3 cm pieces. Strain the cooking liquid and reserve.
Melt the butter and sauté the onion until soft and glassy. Add mushrooms and cook for 3 minutes. Stir in flour and cook until it becomes nut brown.
Stir in milk and stock. Bring to the boil. Add sago, ham, mustard, parsley and chicken.
Simmer for about 10 minutes. Season with black pepper. Cool completely.
Roll out a third of the pastry and use to line 1 large, or 2 small, deep ovenproof dishes.
Pour the cold filling into the pastry shell. Cover with the remaining pastry, cut a hole in the centre to allow steam to escape and decorate with pastry leaves. Glaze with egg yolk just before baking.
Bake in a preheated oven at 200 ºC for 20 minutes, reduce heat to 180 ºC and bake for a further 20 minutes.