Preheat the slow cooker if necessary; see the manufacturer's instructions. Bring a saucepan of water to the boil, add the potatoes and cook for 4–5 minutes or until almost tender. Drain and reserve.
Heat the oil in a frying pan, add the onion and fry, stirring, for 5 minutes or until softened and just beginning to turn golden. Stir in the peppers and garlic and fry for 2–3 minutes. Mix in the harissa, tomatoes, tomato purée, stock and a little salt and pepper and bring to the boil.
Tip the potatoes into the base of the slow cooker pot. Rinse the fish well, drain and arrange in a single layer on top of the potatoes, then cover with the hot tomato mixture. Cover with the lid and cook on low for 5–7 hours or until the potatoes are tender and the fish flakes when pressed in the centre with a small knife.
Spoon into shallow bowls and serve with warmed pitta breads, if liked.