Pre-heat the grill on high and heat the oil in a large non-stick frying pan.
Crack 3 eggs per person into a large bowl, whisk and add in the chopped parsley, salt and pepper and give the mix a final whisk. Add the onion to the pre heated frying pan and fry on a high heat for about 4-5 mins until brown. Add in the chopped tomatoes and olives and cook for a further 1-2 mins.
Turn the heat down from high to medium and add in the egg mixture. Cook in the pan, stirring as the eggs begin to set (partially cooked, but runny in places). This should only take about 2 minutes. Roughly crumble the feta in the pan, then place under the pre-heated grill for 5-6 mins until the omelette is golden on top. Remove from the grill and serve from the pan.