Heat oil in a large saucepan over medium heat. Add onion and garlic; cook 6 minutes or until tender, stirring frequently. Stir in ginger, paprika, salt, cumin, and pepper; cook 1 minute. Add 3 cups water, lentils, chickpeas, and tomatoes; bring to a boil.
Cover, reduce heat, and simmer for 30 minutes or until the lentils are tender, stirring occasionally. Stir in lemon juice and harissa. Garnish with chopped fresh cilantro, if desired.