Place the whole chicken in the large saucepan and cover with cold water.
Add the bouquet garni, bring to the boil, reduce the heat and simmer gently for an hour and a half.
Remove the chicken to a chopping board and pierce it between the breast and leg to check that no blood is released.
Remove the meat from the carcass and chop into chunks. Place in the bowl and set aside.
Place the remaining stock on the heat and boil until reduced to about 1.5 litres.
Heat the oil in the casserole and add the onions, carrots, parsnips and turnips or swede. Cover and sweat the vegetables on a moderate heat for 5 minutes or so.
Add the leeks and continue to cook for a further 5 minutes uncovered.
Add the parsley and seasoning and pour in the stock through a sieve.
Simmer for about half an hour until the vegetables are soft.
Add the oats, cabbage and chicken and cook for a further 10 minutes.
