Cook chicken: Place the chicken breasts in a pot, cover with water. Add peppercorns, bay leaves, allspice. Bring to a boil, add a little sea salt and the stock cube, turn the heat down, simmer for about 10 minutes or until the chicken breasts are cooked through. Add the frozen peas during the last 3 minutes of the cooking time. If using leftover chicken remove the skin and the bones (if necessary), chop the meat. Defrost the peas and set aside.
Chop chicken: Lift the chicken from the water, let it cool for a about 5 minutes. Chop into 2 cm/ 0.7 inch cubes. Chop the ham into small cubes as well. Drain the peas reserving the cooking water. Discard the spices.
Preheat the oven to 180 degrees Celsius/ 360 degrees Fahrenheit.
White sauce: Chop the onion finely. Melt the butter in a medium saucepan. Cook the onion until golden. Sprinkle the flour on top and stir gently until the flour gets lightly golden as well. Gradually add 200 ml/ 6.7 fl. oz/ scant 1 cup of the reserved cooking water and the milk while whisking continuously. Turn the heat down and stir until the desired thickness is achieved, about 8 minutes. Add salt, pepper, and nutmeg to taste.
Combine: Take off the heat and stir in the chicken, ham, and peas. Adjust the taste again, adding more salt, pepper, and nutmeg if necessary. Pour the mixture into a pie dish of about 28 cm/ 11 inches diameter.
Cover with pastry: Unroll the puff pastry disc and cover the pie with it. Seal the pie by pressing the pastry disc firmly on the sides of the dish. I used a fork and pressed and crimped all around the pie dish. Trim off the excess pastry. You could use this excess to cut out leaves or flowers to decorate the pie with. It is not a necessary step.
Egg wash: Mix the egg yolk and the milk in a small bowl. Brush the pastry with this mixture. Place the leaves/flowers on top of the pie and brush them with egg as well.
Bake: Make a small slit in the form of an X in the middle of the pie and some simple slits two or three times around the edges of the pastry. Bake for about 20 minutes until golden brown.
Notes
Frozen puff pastry can be used instead. Defrost and roll according to the packet's instructions.