Preheat oven and a flat tray to 230°C fan forced. Place mushrooms stalk side up on board. Sprinkle over the green onions then crumble over the feta. Season with pepper and spray lightly with oil. Place mushrooms onto the hot tray, squeeze over the lemon and bake 8-10 minutes or until tender.
Heat a medium frying pan over medium heat until hot. Lightly whisk the eggs, water and parsley together, season with pepper. Add half the butter to the frying pan. When foaming, add half the egg mixture. Tilt the pan until the egg coats the base of the pan. Reduce heat to low and cook for 2-3 minutes or until almost set.
Shake the pan to ensure the omelette has not stuck to the base. Fold the omelette into quarters and slide onto a serving plate, cover to keep warm while making the remaining omelette.
Serve omelette with mushrooms and toasted grain bread.