Start by making the green herb sauce, place the parsley and mint on a board and use a nice sharp knife to chop it all up. Add the capers, mustard and grated garlic and carry on chopping until it’s all fairly fine.
Spoon into a bowl and add the vinegar and olive oil and season with salt and pepper to taste. It should have a loose-ish consistency, a bit like a dressing, so add a splash more oil or a tiny dash of water if it’s a bit thick.
Scrub the New potatoes to remove any grit or soil. Halve the big ones, then arrange them over a large baking tray in a single layer. Trickle over 2 tbsp of olive oil and season them well with salt and pepper.
Place the potatoes into a hot oven, about 200°C, and roast them for 15 to 20 minutes. Remove the tray from the oven and scatter over the bay leaves, sliced red onions and lemon zest. Turn everything together and return to the oven for a further 12–15 minutes or until the potatoes are tender and golden.
Cut the mackerel fillets in half, trickle them with the remaining olive oil and season them all over with a little salt and pepper.
Place the fish on top of the potatoes and return to the oven for about 5 minutes; this should be enough to cook the mackerel through.
Remove the tray from the oven and dot all over with the green herb sauce. Serve at once with a nice crisp green salad.