Preheat the grill. Rub the mackerel with a little olive oil, season, and lay on a tray, skin-side up. Put the olive oil and butter into a hot frying pan. Once foaming, add the potatoes, onions & thyme sprigs. Cook for 12-15 minutes until crispy & golden and finish with salt, pepper and the vinegar. Meanwhile, grill the mackerel for 4-5 minutes until the skin has blackened slightly and the flesh is cooked. Serve the grilled fillets on top of the crispy potatoes along with a crisp green salad.