Heat oil in a medium saucepan over medium heat. Add onion; cook 5 minutes or until tender, stirring occasionally. Add lentils, ginger, pepper, and cinnamon; cook 10 minutes, stirring occasionally. Stir in 1 cup water and broth; bring to a boil. Reduce heat, and simmer 10 minutes. Add remaining 1 cup water and juice, and cook 30 minutes or until thick, stirring occasionally. Stir in salt. Remove cinnamon stick; cool lentil mixture completely. Spread about 1/2 cup mixture over each flatbread. Spread 1 tablespoon yogurt over each serving; divide cucumber evenly among flatbreads. Roll up. Cover and chill.