Heat the olive oil in a large heavy-bottomed saucepan. Add the onions and garlic and fry gently until softened. Add the celery, red pepper, leeks and mushrooms, gently stirring together.
Add the stock, soy sauce and tomatoes and bring to the boil then turn down to a gentle simmer. Add the carrots, butter beans and the whole chicken thighs and leave all to simmer gently for approximately one hour.
To make the rice, bring to the boil a pan of water seasoned with a pinch of salt. Add the rice and cook according to packet instructions.
Take out one of the chicken thighs and gently break it open. Check that the meat is all thoroughly cooked (the meat will be white, not still pink). When cooked, remove the chicken thighs from saucepan and remove the bone. The chicken should literally fall off the bone. If desired, cut into bite-sized pieces and return to the pan.
Drain the rice and place into bowls. To serve, generously ladle casserole onto bowls of rice.