Place a large Dutch oven or soup pot over medium-high heat. Let it heat up for 2 minutes or so, then pour in the olive oil. When the oil sloshes around the pot easily, add the onion, carrots, and celery. Cook, stirring frequently, for about 5 minutes or until the onion is soft and translucent.
Sprinkle the vegetables with the cornstarch, stir for 1 minute, and stir in 1 teaspoon of salt, the black pepper, turmeric, cumin, paprika, cinnamon, and ginger. Stir for 1 minute, add the rice and lentils, and cook, stirring constantly, for 2 more minutes. You'll notice a brown coating on the bottom of the pot; that's OK! Just keep stirring for the full 2 minutes so the spices do not burn.
Pour in the broth and tomatoes and add most of the parsley and cilantro (set aside ¼ cup of the chopped fresh herbs for garnishing later). Increase the heat to high and use a wooden spoon or spatula to scrape up all the brown bits from the bottom of the pot. As soon as the liquid begins to bubble, reduce the heat to medium and partially cover the pot with the lid slightly ajar. Cook for about 15 minutes, stirring occasionally, until the lentils and rice are tender.
Stir in the chickpeas, lemon juice, and remaining ½ teaspoon of salt. Taste and season with more salt, lemon juice, or black pepper as desired.
Portion the hot soup into bowls, top each with some of the reserved fresh parsley and cilantro, and serve with a ramekin of harissa sauce for those of us who like some extra heat.