For a non-vegetarian version why not ripple over slices of ham or bacon before scattering over the feta, then grill until crispy.
Heat the grill to high. Whisk the eggs in a large bowl with the chopped parsley, pepper and salt, if you want. Heat the oil in a large non-stick frying pan, then fry the onion wedges over a high heat for about 4 mins until they start to brown around the edges. Throw in the tomatoes and olives and cook for 1-2 mins until the tomatoes begin to soften.
Turn the heat down to medium and pour in the eggs. Cook the eggs in the pan, stirring them as they begin to set, until half cooked, but still runny in places – about 2 mins. Scatter over the feta, then place the pan under the grill for 5-6 mins until omelette is puffed up and golden. Cut into wedges and serve straight from the pan.