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