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.
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 and peppers in a splash of olive oil until golden brown and cooked. Remove from the pan and set aside.
In the same pan, fry the mushrooms until golden brown then add the garlic and fry for another 30 seconds then tip into the same bowl as the peppers.
Poach the peas in boiling water until de-frosted then drain and set aside.
In a large pot, melt the butter then whisk in the flour to make a roux.
Slowly whisk in the stock, milk and cream and allow the sauce to simmer until thick and smooth. You might not need all of the liquid.
Shred the chicken, discarding any skin, boned and cartillage. I break the chicken meat into large chunks as I don't like it when it loses its texture.
Add the chicken and vegetables to the white sauce then stir well. If the sauce is too thick at this stage, simply add a splash of milk/stock.
Add the lemon juice and zest and allow to simmer gently for 10 minutes.
Season to taste then serve with rice/mashed potatoes.