Put the chicken and stock into a saucepan over a medium heat. Cover, bring to a simmer and cook for 8-10 minutes until the chicken is cooked through. Transfer the chicken to a separate bowl, reserving the liquid in a measuring jug to add to the sauce later.
Heat the oil in the same pan, add the onion, leek and carrots and saute until soft, about 8 minutes. While the vegetables are cooking, shred the chicken into small pieces, and put them into a 22cm x 22cm baking dish. When the vegetables are ready, add them to the dish and stir well.
Add the butter to the same pan and melt it over a low heat. Add the flour, stir well and cook for 1 minute. Gradually whisk in the chicken stock from the measuring jug (about 460ml), the milk and thyme. Bring to the boil, then turn down the heat and simmer, stirring until the sauce thickens enough to coat the spoon - another 1-2 minutes. Pour the sauce over the chicken and vegetables, add the peas and parsley and stir well to combine.
Lay the pastry sheet over the top stretching it a bit if necessary, and trim to fit, crimping the edges. Cut a few slits to vent the steam. Place the dish on a baking sheet and bake for 30-40 minutes until the crust is golden and the filling is bubbling.
Our classic mash-topped Chicken Pie is the ultimate toddler comfort food. This tasty pie contains a delicious cheese sauce with chicken, sweet potato and peas, topped with a vibrant potato and sweet potato mash. To make the sauce really tasty, and gently introduce new flavours, we've added a sprinkle of thyme and nutmeg. Just don't tell our chefs we've told you their secret ingredients!