Set oven control to broil. In medium bowl, mix Cucumber Sauce ingredients; cover and refrigerate until ready to use.
Cook spinach as directed on box; cool slightly. Squeeze to drain well. In large bowl, mix cooled spinach, turkey, 1/4 cup yogurt, the oregano, garlic salt and pepper. Shape into 5 oval patties, each about 1/2 inch thick. On broiler pan, place patties. Broil with tops about 5 inches from heat 10 to 12 minutes, turning once, until thermometer inserted in center of burgers reads 165°F.
Place 2 tomato slice halves in pita pocket halves; place burgers over tomato slices. To serve, top each burger with about 3 tablespoons sauce.