Get your largest saucepan and fill halfway with water. Set on low heat. Dissolve stock cubes in the amount of water that is recommended on the packet and add to water in the pan. Add onions and mushrooms to the pan.
Grill chicken pieces until juices run clear when pricked with a fork. When cooked remove skin and bones and chop meat finely. Add meat to the pan. Add the tinned crab. Liquidise one tin of sweetcorn and add with the second tin. Season with soy sauce and black pepper.
Cook for about 1 hour as gently as possible.
Make a beurre manié: blend flour and butter together to make a paste. Then add small amounts of soup to it until it has a sauce-like consistency. Add to soup, stir and cook for about 10 minutes. Serve hot.