Saute the onion in olive oil and butter on medium heat, for about 3-4 minutes. Stir in garlic and spinach and saute for another 3-4 minutes. Add salt and pepper to taste.
Transfer spinach onto the plate, and place in a cold place for a few minutes.
Meanwhile, prepare the egg mixture by whisking the rest of the ingredients together.
Now, it’s time to remove extra liquid from spinach. Simply take a handful of spinach, and try to squeeze the juices out with your hands. Transfer dry spinach into another bowl.
Add egg mixture to the spinach. Stir. Divide the filling into 4 parts. Spread 1/4 of the filling onto a half of tortilla and fold the other half over. Fry on ungreased frying pan, on low heat for 4 minutes on each side.