Heat oil in a 4-quart stockpot over medium heat. Add onion and sautéed until translucent. Add cumin and curry powder. Cook for one minute. Add lentils, sweet potatoes, broth, and tomatoes. Bring to a boil. Simmer for 25 minutes of until lentils and sweet potatoes are soft. Add drained and rinsed chickpeas. Heat thoroughly. Serve with seasoned yogurt and naan bread if desired. Serves 4.