HEAT grill to medium heat or heat broiler. Combine oregano, cumin and garlic powder in large bowl. Add lamb and mix well. Form into 4 patties, about 3/4-inch thick. Season with salt and pepper. Grill or broil about 5 minutes on each side or to desired degree of doneness.
COMBINE mint sauce, yogurt, cucumber, green onion and sugar in small bowl. Stir until mixed. Split open each pita half, forming a pocket. Place burger in each pocket. Top with spinach and yogurt mixture.