Combine steak, olive oil, vinegar, tzatziki, and salt in a bowl or ziploc bag.Marinate for 1 hour or up to 12 hours.
Heat a saute pan over medium high heat. Add 2 tsp olive oil and cook steak to desired doneness; 3-4 minutes per side for medium rare. Let steak rest for 10 minutes then thinly slice.
Warm pitas by wrapping in tin foil then placing them in a 350F oven for 8-10minutes.
Make a tzatziki dip by combining the yogurt, sea salt, tzatziki seasoning, and feta cheese.
Assemble by spreading 1 Tbsp of tzatziki dip on pita, then adding steak slices, a spoonful of tomatoes, onion, and a small handful of spinach.