Steps
1. Preheat the oven to 350 degrees F.
2. Remove the stem from the mushroom and clean the inside of the mushroom well with a damp cloth.
3. Rub the mushroom with some olive oil, generously season with salt and pepper, and place it hollow side up on a baking tray.
4. Heat the remaining oil in a pan. Add the onion and cook for 1-2 minutes.
5. Add the cooked rice, nutmeg, salt and pepper, and cook for two more minutes.
6. Transfer the contents of the pan into the portobello mushroom.
7. Bake in the preheated oven for 20 minutes.
8. Serve warm.