Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the onion and fry, stirring, for 5 minutes or until lightly browned.
Add the carrots and swede to pan with the red pepper, garlic, beans and tomatoes. Mix in the harissa, turmeric and ginger, then pour on the stock and season with salt and pepper. Bring to the boil, stirring.
Spoon the mixture into the slow cooker pot and press the vegetables beneath the surface of the stock. Cover and cook on low for 6–8 hours or until the root vegetables are tender.
Stir in the okra, cover and cook on high for 15–20 minutes or until the okra are tender but still bright green. Garnish with mint leaves and serve with couscous soaked in boiling water and flavoured with olive oil, lemon juice and sultanas.