Soak the rice in water for 15 minutes. Cut the onions, leeks and mushrooms. Heat butter in a deep pot and sauté the onions first, then the leeks and finally add the mushrooms, and then the cream. When softened, add salt, black pepper and nutmeg. Drain the rice and put it in with the other products.
Stir for half a minute and pour into a greased baking tray. Sprinkle it with the crushed broth cube; if desired, add more spices and top up with a few cups of hot water (almost to the top of the tray).
Put the rice to bake in a preheated 392°F (200 °C) oven until all the liquid evaporates.