Put the potatoes in a pan of salted water and bring to the boil. Simmer for 5-10 minutes, or until tender. Drain, return to the pan and leave to dry. Shake the pan to fluff up the potatoes.
Heat the butter and 2 tbsp of the oil in a pan over a medium heat. Gently cook the onion for 5 minutes, until softened, adding the garlic and cumin for the last 30 seconds. Tip in the potatoes, increase the heat and cook for 5 minutes, turning, until lightly golden. Add the spring onions and cook for 5 minutes more; season well.
Meanwhile, score the skin of each mackerel fillet. Turn them over and rub 1 tsp of harissa into the flesh of each one (avoid getting any on the skin or it will catch during cooking). Heat the remaining oil in a pan. Cook the fish, skin-side down, for 3-4 minutes, then turn and cook for 30 seconds more or until cooked through.
Scatter the parsley over the potato mixture and divide between plates. Top with the fish. Serve with lemon wedges.