In a small baking dish, arrange tilapia fillet and sprinkle with lemon juice, salt and pepper. Scatter green onion, mushroom slices and olives over top. Spoon tomatoes over, adding some of the tomato juices if desired. Sprinkle with oregano. Top with crumbled feta.
Cover dish, and bake for approximately 15 minutes, or until fish is opaque and flakes easily with a fork.