1. Bring the water and a generous sprinkling of salt to the boil in a large, heavy-based saucepan. Place the chicken in the water,
add the peppercorns and boil in an open pan for about 20 minutes, scooping off the scum as it forms.
2. Quarter the onion. Trim and wash the bouquet garni, if necessary, and add it to the chicken with the onion. Cover the pan and simmer gently for 1 1/2 hours.
3. Trim and wash the cauliflower and separate it into florets. Halve the leeks lengthways, wash and slice. Scrape and slice the carrots. Clean the sprouts and cut larger ones in half.
4. Lift the chicken out of the pan and strain the stock into a large saucepan. Add the vegetables to the stock and cook in a covered pan for 25 minutes.
5. Boil the vermicelli in salted water for 8 minutes, then drain. Dice the chicken and add it to the cooked vegetables with the vermicelli. Season to taste with soy sauce and sprinkle with parsley.