Thinly slice the onion.
Heat a little oil in a non-stick frying pan and cook the onion until soft.
Whisk the eggs in a bowl, then add the fried onions. Crumble the feta over it. Season with salt and pepper. Combine.
Turn on the grill in the oven.
Heat some more oil in the pan and pour in the egg mixture, spreading it out evenly. Turn down the heat and cook until the top sets.
Put the pan under the grill for 2/3 minutes, until the top get golden brown. Alternatively, flip the omelette over and cook for a couple of minutes.
Transfer it a plate and sprinkle with chopped herbs.
Serve warm or at room temperature.