Place all stock ingredients in a heavy-based pan or stockpot, then bring to the boil and simmer for 30 minutes.
Carefully lower chicken pieces into stock, skin-side down, and cover with a cartouche (see below) to keep them fully submerged. Poach chicken gently for 15 minutes. There should be no more than an occasional ripple breaking the surface; adjust temperature, if necessary, to ensure stock does not reach simmering point again. At the end of the poaching time, remove pan from stove and allow chicken to steep in stock for 5 minutes to complete the cooking process. Using tongs, gently remove chicken from stock, being careful not to tear skin. Place chicken on a tray to cool slightly.
Meanwhile place pan back on stove, then add mushrooms and bring stock back to the boil. Simmer until reduced by approximately half – this should take about 20 minutes.
Using a cleaver or heavy knife, separate chicken portions into legs and thighs, then chop into thick, even slices. Arrange chicken on a serving platter, then drizzle with hot stock, spoon on mushrooms and serve.
To make a simple cartouche, just tear a piece of baking paper or greaseproof paper roughly the size and shape of your pan and press it down onto the surface of the liquid, covering all the ingredients.
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