Mash the potatoes into a puree. Add the eggs to it and stir until homogenized. Add the onion and parsley to it, finely chopped. If the mixture is too thick, you can add a little milk. Add crumbled feta cheese, some salt and other spices to taste.
Optionally, you can grate a little feta cheese on the side you intend to serve. Serve the omelette with a tomato and feta cheese.