Preheat the oven to 180°C / 350°F
Place the chicken breasts in a saucepan with the skimmed milk, the chicken stock and a couple of sprigs of fresh thyme.
Heat on a low to medium setting until the liquid starts to gently simmer.
Cover the saucepan and cook for 15 minutes making sure that it never goes above a gentle simmer.
Remove from the heat and carefully remove the chicken breasts and the sprigs of thyme from the liquid. Set the liquid aside for use later and discard the thyme.
Using two forks, shred the chicken breast (or cube if you prefer!) and set aside.
Mist the bottom of a large skillet with some calorie controlled spray oil and heat over a medium heat.
Add the chopped onion and saute for 4 - 5 minutes until they have softened slightly. You do not want the onions to brown.
Add the minced garlic and cook for a further 1 minute (making sure that you stir it to prevent it from sticking to the bottom of the pan).
Add the white wine vinegar and use a wooden spoon to scrape any browned bits of onion or garlic from the bottom of the pan.
Once the vinegar has evaporated you can add the chopped carrots and celery.
Cook for 3 - 4 minutes, stirring often. Remove from the heat
In a small pot mix the cornstarch (cornflour if you are from the UK) with a couple of tablespoons of water. You want it to have the consistency of a thick yogurt.
Return the poaching liquid to the stove and heat to a gentle simmer.
Whisk in the cornstarch mix into the poaching liquid and increase the heat a little.
You are wanting the sauce to thicken and for any starchy cornflour taste to cook off. However, you do not want to overcook the sauce at this stage as cooking it for too long at too high temperature can lead to the sauce thinning out again.
Remove from the heat and stir in the shredded chicken, some freshly ground black pepper, the chopped cooked vegetables and the frozen peas.
Stir in the nutritional yeast if using. This is definitely not a required ingredient but if you like the nutty/cheesy flavour it lends to foods then it is a great addition.
You may also like to add some salt but this will depend on the saltiness of the stock cubes that you used. If in doubt, leave out the salt and just season it once it is on your plate!!
Tip the pie filling into a 1.5 quart oven proof dish approximately 8inch (20cm). The filling should almost fill the dish to the top.
Set aside whilst you prepare the puff pastry topping.
How you do the pie top is dependant on the puff pastry brand you are using as some sheets are larger than other & some are more SmartPoints than other. See detailed notes in the main body of text!
Use a sharp knife to cut 8 'ribbons' of pastry that are just a little longer than the top of your dish.
Use a little of the egg wash around the edge and lip of the dish and top with the puff pastry ribbons. Using the egg wash around the lip and edge of the dish means that you can stick the puff pastry to the edges of the dish which should prevent the pastry from shrinking and falling into the pie filling when it's cooking.
Use the egg wash to brush the top of the pastry before placing in a preheated oven for 35 - 40 minutes or until the puff pastry is golden brown that the filling is bubbling hot.