Heat a greased medium size skillet over medium heat. Add the shaved steak, marinade (if using) and spinach. Saute the steak and spinach 3-4 minutes until the steak is cooked to you liking and the spinach has wilted.
Remove the steak and spinach from the skillet and wipe clean.
Assemble the Steak Omelet
Heat the the skillet to medium heat. Spray well with cooking spray or melt 1/2 tablespoon of butter.
Pour in the beaten eggs and cover with a lid. You do not want your eggs to bubble up. If they are, reduce the heat to medium-low.
Cook eggs with the lid on for 1-2 minutes or until the eggs are almost set.
(If they are not set, give it another 30 seconds to one minute covered.)
Add the steak, greens, feta, and tomatoes to one side of the omelet and fold the other side over the toppings.
Let it cook for approximately 20 more seconds, then carefully remove it from the skillet.