Preheat the oven to 180°C.
Finely chop the onion and garlic. Pat-dry the roasted pepper and thinly slice it. Remove the stems of the mushrooms with a spoon and chop them roughly.
Heat the oil in a saucepan, then add the onion, mushroom and garlic and cook for about 5 minutes until soft.
Add the oregano, pepper and stir for a couple of minutes. Add the rice and stir (you can add a drop of liquid - passata, stock etc - if it's too dry.
Turn off the heat, then add the olives and season to taste.
Crumble the feta into the cooked rice. Add the chopped parsley and stir.
Stuff the mushrooms with the rice filling and bake for 12–15 minutes.
Serve with a crispy salad.