Fill a large stockpot with water and place the chicken into the pan.
Cut the green leaves off the leek, discard any outer withered leaves, and place the rest in the pot with the carrot, bay leaves, peppercorns, and salt.
Place on the cooktop over medium to high heat and bring the water to a boil. Reduce the heat to a low simmer, cover the pan and cook for 45 minutes.
Remove the chicken and strain the cooking water (stock) into a large jug. Set aside to use later. Allow the chicken to cool before removing the meat from the carcass.
Preheat the oven to 190°C.
Wash the remaining white part of the leek and slice finely.
Place the diced mushrooms in a steamer basket with ½ cup of water in the pan below. Steam the mushrooms for 15 minutes. Retain the water to use later.
Heat the olive oil in a large frypan and add the leeks. Sauté for 3 - 4 minutes until soft.
Add the flour and stir through the leeks for about 1 minute. Stir in the water from the mushroom pan and continue stirring until well combined.
Take 1 ½ cups of the stock used to cook the chicken and slowly stir into the pan to make a smooth creamy sauce. Add the lemon juice, crème fraiche and season to taste..
Fold through the steamed mushrooms and chicken and transfer to a pie dish.
Defrost a sheet of puff pastry. Slice into lengths approx. 2 cm thick and arrange in a lattice design over the top of the chicken filling.
Brush the pastry with beaten egg and place in the oven at 190 C for 25 - 30 minutes until the pastry is golden.
Serve with accompanying green veggies.
Use the remaining chicken stock to make soup.