Heat oven to 400°. Heat grill or grill pan to medium heat.
In a medium-size bowl, stir together the cucumber slices, tomato, yogurt and salt. Set aside.
In a large bowl, combine ground chicken, spinach, feta cheese, oregano, bread crumbs, onion powder, salt and pepper. Stir together until blended. Shape into 4 equal-size patties. Coat patties with nonstick cooking spray.
Grill burgers for 12 minutes, flipping once, or until instant-read thermometer inserted into burger registers 160°. Meanwhile, bake pitas at 400° for 8 minutes. Place one burger on a pita round. Top burger with about 1/4 cup tzatziki, if desired, and a second pita. Repeat with remaining ingredients; serve extra tzatziki on the side.