Put the dried mushrooms into a bowl, pour over the water and leave to soak for about 2 hours.
Preheat the oven to 170°C/325°F/gas mark 3. Heat a little fat or oil in a large frying pan over a fairly high heat and fry the pheasant joints in 2 batches just until browned on each side. Transfer the joints to a large casserole dish. Using a sharp knife, slice the lemon grass across very finely. Cut open the chilli lengthways under running water and discard the seeds and stem. Peel the ginger and the garlic. Then chop the chilli, ginger and garlic up together finely and scatter, together with the lemon grass, over the pheasant joints in the casserole dish. Add the orange rind, blueberries and orange and lemon juice. Strain the soaking water from the mushrooms into the dish and then press the mushrooms down between the pheasant joints. Season with salt and cover the casserole. Cook on the centre shelf of the oven for about 2 hours, then pour the juices into a saucepan. Cover the casserole to keep the pheasant joints warm, or put it in a low oven if leaving for long.
Mix the arrowroot or cornflour with a little water in a cup until smooth and stir into the cooking juices. Bring to the boil, stirring all the time, and bubble, still stirring, for about 2 minutes. Remove from the heat and cover the saucepan, if making in advance. Just before serving, reheat the sauce and roughly chop the coriander leaves. Then pour the sauce over the pheasant and scatter with the coriander leaves.