- Either the night before or at least one hour before serving, make the tzatziki sauce.   
- In a food processor, combine yogurt, chopped cucumber, 1 tbsp olive oil, 2 tbsp lemon juice, 1 tsp of the lemon zest (should be about half of the zest), 2 tsp fresh chopped dill, salt and pepper.   
- Pulse until blended but still has some texture to it (I like to have some small pieces of cucumber remaining).   
- Chill for at least one hour or overnight to blend flavors 
- Toss steak with remaining lemon juice and zest.  Allow to marinate in the refrigerator in a covered bowl for about 10 minutes while you prep your other ingredients for assembling quesadillas. 
- Heat remaining tbsp of olive oil a skillet over medium-high heat. 
- Drain steak and pat dry with a paper towel.  Cook steak in heated skillet for 2-3 minutes per side or until cooked to your desired doneness.  Since we are working with thin strips, it will cook quickly. 
- Remove steak from the pan and set aside.   
- Add the fresh spinach to the pan and cook until just wilted, stirring frequently.  Set spinach aside. 
- Now you're ready to assemble your quesadillas.  Layer ingredients on one side of the tortilla starting with a layer of cheddar cheese on the bottom and finishing with the feta on top.   
- Spray skillet with an olive oil cooking spray and cook quesadillas on medium-high heat for a couple of minutes each side until lightly golden and cheese is melted.   
- Serve immediately with a side of the tzatziki for dipping