Add the saffron to the stock in a jug, and leave to infuse. Heat the oil in a large saucepan and add the chicken. Brown all over for 5-6 minutes and then remove and set aside. Add the onion, and cook for 2-3 minutes, until beginning to soften. Stir in the garlic and cook for another minute, then season.
Add the carrot, celery, harissa paste and spices to the pan. Cook for 5-6 minutes, stirring so everything is combined and the vegetables are just tender.
Pour in the infused stock, chopped tomatoes and add the chickpeas. Stir everything together, season and then bring to the boil. Add the chicken, turn down to a simmer and cover with a lid. Season, simmer gently for 20 minutes, before removing the lid and cooking for another 10 minutes, until the soup has reduced slightly.
Spoon out the chicken thighs and shred with 2 forks. Return to the soup, stir through and then divide between 6 bowls. To serve, add a spoonful of crème fraîche and sprinkling of parsley (optional).