Put the potatoes into a medium saucepan and cover with salted water.
Bring to the boil, then turn down to a gentle simmer and cook for 10-15 minutes, until the potatoes are tender.
While the potatoes are cooking, heat the butter and oil in a pan.
Add the onion and garlic and sauté for a couple of minutes until soft.
Stir in the cumin and cook for a further minute.
Drain the potatoes and stir them into the onion mixture.
Season well. Add the spring onions, stir and cook for a couple more minutes until the onions are soft.
Meanwhile, preheat the pan.
Brush the mackerel fillets with harissa paste and pan-fry them skin-side for 3 to 4 minutes. Then turn the fish and cook the other side for 30 seconds more, until cooked through.
Place the chopped parsley over the potato mixture, top with the fish.
Serving Suggestions: Serve with lemon wedges. This recipe goes well with seeded flatbreads.