Heat a little oil or margarine in a pan and fry the mushrooms for a few minutes.
Add the spinach and let it shrink.
Season this vegetable mixture with a little pepper and salt.
Mix the eggs in a bowl with a splash of milk.
Heat a little oil or margarine in a frying pan and pour in half of the egg mixture.
Bake the omelet for 2 to 3 minutes on one side and then turn it over and bake the other side for another minute.
Repeat this with the rest of the egg mixture and make a 2nd omelet.
Spoon some of the vegetable mixture onto the omelet in one line.
Put some strips of grilled bell pepper on top and crumble some feta over it.
Roll the omelet and place on a plate with some arugula and serve with the tzatziki.