In a small skillet, heat oil over medium-high heat. Add onion, oregano and pepper flakes; cook and stir until onion is tender, 8-10 minutes. Transfer to a 5- or 6-qt. slow cooker., Add chickpeas, lentils, olives and paprika; stir in broth and tomato sauce. Cook, covered, on low 8-10 hours, until lentils are tender. Stir in spinach. Top servings with yogurt.