Heat the oil in a non-stick frying pan. Add the onions, season with salt, cover and cook for approx. 7 mins. Allow the onions to cool slightly, set the pan aside.
Make a cut from the centre to the outside of each tortilla. Top ¼ of the tortillas with onions, tomatoes, cheese and spinach.
Fry one tortilla in the same pan for approx. 2 mins., fold, turn, fry for a further 2 mins. Repeat the process with the second tortilla.