Wash and clean the peppers of their stems. Peel and chop the onions. Put the oil in a deep pan to heat and braise the onions.
Wash the mushrooms and chop them, add them to the onions to braise as well; they will shrink.
Clean and wash the rice, add it to the mushrooms and stir so it doesn't stick to the bottom.
Add the tomatoes, either mashed or cut, to the rice. I cut them.
Pour in 1/2 cup water and cook until the rice is semi-cooked, it still needs to be hard but to have taken in water and juice from the tomatoes.
Season with black pepper, salt and parsley, which you've washed and chopped finely. Leave aside a little parsley for decoration if desired.
Fill the peppers using a teaspoon and arrange them in a tray.
Pour in water to cover them and put to bake in a preheated 392°F (200 °C) oven for 40-45 min.
Once they're ready, sprinkle with parsley and serve.