Heat olive oil in a large heavy-bottomed saucepan and soften onions and garlic.
Stir in celery, red pepper, leeks, and mushrooms.
Add 1.75 litres of stock, soy sauce, and tomatoes then bring to a boil.
Add carrots, butter beans, and chicken thighs then simmer for one hour.
Boil a pan of salted water and cook 285g of rice according to instructions.
Remove cooked chicken from the pan and strip meat from the bones.
Cut chicken into bite-sized pieces and return it to the saucepan.
Drain the rice and serve the casserole generously over it in bowls.