Halve, peel and finely chop the onion. Heat half the butter in a flameproof casserole, add the onion and fry until translucent. Add the bulgur wheat and cook, stirring frequently, for 3 minutes.
Add the stock and bring to the boil, reduce heat, cover and simmer gently for about 10-15 minutes, until all the stock has been absorbed.
Meanwhile, clean and finely slice the mushrooms. Rinse and dry the parsley and chop it finely, set aside. Heat the olive oil in a frying pan, add the mushrooms and fry until softened and lightly browned. Season with salt and pepper. Pour the mushrooms and any juice on top of the partially cooked bulgur wheat, recover the casserole and leave to continue cooking.
Add the flaked almonds to the frying pan and shake them over a moderate heat for a minute or two, then add the chopped hazelnuts and cook both until lightly browned.
When the stock is absorbed and the wheat is cooked, stir in the remaining butter, toasted nuts and parsley. Taste for seasoning and serve straight from the casserole.