Season the chicken with sea salt and pepper as early as possible.Heat the oil in a large saucepan over medium heat. Add the chicken pieces in 2 batches and brown on both sides. Remove the chicken from the pot and set aside.Add the onion, celery and carrot and cook until softened. Add all the spices and cook for 1 minute, stirring constantly to prevent scorching. Add the tomatoes with their juice, chickpeas and 1½ teaspoons of sea salt. Cook until fragrant.Return the chicken to the pot with any accumulated juices. Add the stock, lentils and brown rice and bring to a boil. Reduce heat to low and simmer, covered for 50 minutes.Add cilantro and parsley and cook uncovered for 5 minutes or until the chickpeas are heated through. Stir in the spinach leaves, if using. Ladle into individual bowls and serve with a wedge of lemon.