Preheat the oven to 200`c. Wash the mushrooms, remove stems, spray with oil based non-stick spray or mist with olive oil, and place stem-side down on baking sheet. Bake 5 minutes, just long enough to dry out some of the water. In food processor, combine mushroom stems, spring onions, olives, feta cheese and breadcrumbs. Pulse a couple of times, and then drizzle in just enough olive oil to moisten the filling so it will slightly stick together. Fill inside of each mushroom cap with filling, pressing down so it stays in. Bake 180 degrees for about 8 minutes, until mushrooms are hot and slightly browned and cheese is partially melted. Serve immediately.