Heat the water in a saucepan with the crumble stock cubes. Bring to a simmer and add the onions and mushrooms.
Grill the chicken thighs until the juices run clear when pricked with a fork. Let cool, remove the skin and bones and dice the meat finely. Add to the saucepan with the tinned crab.
Blend one of the tins of sweetcorn and add to the soup with the second tin. Season to taste with soy sauce and black pepper. Simmer very gently for 1 hour.
Blend the butter and flour together to make a paste. Add small amounts of hot soup to it until it has a sauce-like consistency. Add to the soup, stir well and simmer 10 more minutes. Serve.