In a large bowl, combine lamb, garlic, dried oregano leaves, onion powder, salt and pepper; mix lightly but thoroughly. Shape into two oval 1/2-inch thick patties. Place patties on rack in broiler pan so surface of meat is 3 to 4 inches from heat. Broil 8 to 10 minutes or until no longer pink, turning once. Carve each patty into thin slices. Place equal amounts of lamb, tomato and onion in each pita half; serve with Cucumber Yogurt Sauce. Serves 4.