1. Heat olive oil in a large skillet over medium-high heat. Add onion, garlic, salt, pepper, and lamb; cook 5 minutes or until lamb is browned and vegetables are tender, stirring occasionally to crumble lamb.
2. Add lentils, cumin, and thyme; stir until seasonings become fragrant. Add 1/2 cup water, tomatoes, and bay leaf; bring to a boil. Cover, reduce heat to medium, and cook 15 minutes. Stir lentil mixture; add remaining 1/2 cup water. Cover and cook 15 minutes or until lentils are tender and mixture is thick (add additional water as needed). Discard bay leaf. Fill each pita half with 1/2 cup lentil mixture. Spoon 1 tablespoon yogurt into each pita half; top with 2 tablespoons cucumber. Sprinkle with mint, if desired.