Grease a small baking sheet or form with some butter. Saute with the remaining butter in a nonstick pan the chopped onions and the pepper.
Remove onions and pepper from the pan and add the tomatoes. Saute them for 2'-3'.
Put in the sheet or the baking form the tomatoes, the peppers and the onion. Add the feta cheese in small pieces.
Beat cream milk or the evaporated milk with the eggs, salt, pepper and the parsley. Pour the mixture in the sheet over the cheese-tomatoe content. Bake the omelette in the oven at 180° - 200° C for 40'-45 '.