Heat the oil in a large pan. Add the celery, carrot, and onion, and fry gently for 5 minutes, until softened but not browned. Add half the minced preserved lemon, 2 tablespoons of the charmoula, the tomatoes, and the stock. Bring to a boil and simmer for 30 minutes. Add the potatoes and simmer for 6 to 8 minutes until tender.