Preheat barbecue or grill. In a mixing bowl combine lamb, yogurt, cumin, chopped rosemary, salt and pepper. Divide into two equal portions and form into two 3/4-inch thick patties. Grill burgers about 15 minutes, turning once. Brush pita bread with olive oil and grill until browned on both sides. To serve, top pita with a burger, onion, feta cheese and olives. Burn rosemary sprigs for show, if you wish.