Heat pan over medium heat and brush it with butter.
Whisk eggs, with salt, pepper in a bowl. Pour egg mixture into the heated pan and swirl until eggs cover entire bottom of the pan. Continue cooking for 1-2 minutes.
Arrange feta cheese, tomatoes, olives, spinach in the middle of the pan. Cook until edges of eggs begin to curl up for 2-3 minutes.
Place spatula underneath omelet, tip pan to loosen and gently fold omelet in half. Remove from heat. Top it up with remaining cheese and serve.
The omelet can be served with assorted sidelines like grilled tomatoes, extra olives or grilled corns. It is entirely based on your personal preferences.
You can use rocket and some capers in this delicious omelet to give it some extra taste to already very delicious omelet.