Melt 2 tablespoons of the butter in an ovenproof saucepan or casserole with lid. Add the onion and garlic and cook briefly without browning.
Add the rice and bay leaf and cook, stirring, about 2 minutes. Add the stock and salt and bring to a boil. Cover closely and bake exactly 17 minutes.
Meanwhile, melt the remaining butter in a skillet and add the chicken livers. Cook, stirring, over moderate heat until the livers lose their red color. Add the mushrooms and cook until most of the liquid evaporates. Do not overcook or the livers will become too dry.
When the rice is cooked, stir in the liver and mushroom mixture, pimientos and truffles.