Whisk together olive oil, lemon juice, zest, garlic and rosemary in a medium shallow baking dish, add lamb and turn to coat. Marinate in refrigerator for 2 hours. Preheat grill. Grill lamb on both sides for 3 to 4 minutes for medium doneness. Let sit 10 minutes and slice into 1/4-inch thick slices.
Tomato Salad: Mix all ingredients together and let sit at room temperature for 30 minutes.
Feta Yogurt Sauce: Place 4 slices of lamb in the center of each pita. Top with tomato salad and drizzle with yogurt sauce.