In a saucepan, add oil over medium heat. Add onions and green peppers and brown slightly for about 3 minutes. Add brown rice and dried oregano, along with 2 cups of low-fat chicken stock to pan and bring to a boil. Cover and simmer for 25 minutes. Preheat the oven to 400 degrees. Lay 6-8 mushrooms on a baking sheet, top down. Remove the stems (if they have them) with a spoon.
Continue to prepare the filling by stirring black olives, cherry tomatoes, feta cheese and parsley into the cooked rice. Spoon equal portions of rice mixture onto the middle of each of the Portobello mushrooms and bake for 12-15 minutes.
Remove mushrooms with a spatula and serve.