In a large stock pot, place the chicken, carrot, celery, leek, onion, garlic and herbs.
Top with cold water and place over medium heat.
Allow to the chicken to simmer for 1 hour-1 hour 15 mins until it's cooked through.
Remove the chicken from the liquid and allow to cool slightly then shred the chicken, discarding any skin, bones and cartillage. I break the chicken meat into large chunks as I don't like it when it loses its texture.
Drain the stock through a muslin-lined sieve, reserving the liquid. (This makes the most delicious chicken stock so freeze the left-overs in freezer-proof bags for later use.)
To make the chicken a la king, sauté the onions, carrot, celery and peppers in a few tablespoons of butter until golden brown and softened.
Add the mushrooms and the garlic and fry for a few minutes.
Add the thyme and flour and stir into the vegetables.
Slowly whisk in the stock and cream and allow the sauce to simmer until thick and smooth. Season to taste.
Stir in the frozen peas and shredded chicken then taste and adjust the seasoning.
If the sauce is very thick, stir in a splash of milk or stock to thin out the texture.
Add the lemon juice and allow to simmer gently for 10 minutes.
Season to taste then serve with rice or side dishes of your choice.