1.Place mackerel in a non-reactive container. Whisk harissa and 120ml oil in a small bowl, pour three-quarters of mixture over mackerel, turn to coat evenly, set aside.
2.Cook potatoes in boiling salted water until tender (10-15 minutes), drain, coarsely crush. Heat 30ml oil in a frying pan over high heat, add potato and cook, turning once, until golden and crisp (3-4 minutes), season to taste.
3.Heat remaining oil in a large frying pan over high heat. Cook zucchini in batches, turning once, until golden (1-2 minutes), add to potatoes with lemon juice and mint, season to taste, toss to combine.
4.Heat a frying pan over high heat, add mackerel skin-side down and cook, turning once, until golden and just cooked through (2-4 minutes each side). Drizzle with remaining harissa mixture and serve with potatoes, zucchini and mint, with yoghurt and lemon wedges to the side.