Take the garlic and beets, drizzle with olive oil and wrap in tin foil. Bake in the oven at 180 celsius for 15 minutes until just softened. Next take the turnips and peel with a peeler and blanch in boiling, salted water for 6-8 minutes until just soft. When the beetroot is cooled, remove the skin and set aside.
Take the chopped shallot and sauté for 1 minute gently, add the beets and cook for a further 2 minutes then add the turnips, parsley and seasoning, set aside and keep warm. Next, mix the zest and juice with the chopped rosemary and mix in with the soft butter, take a mackerel fillet, season on both sides and cook skin side down a non-stick frying pan for 2 minutes. Turn over and add the flavoured butter and cook for a further 2 minutes. To serve, pile the hash on a plate, stack the mackerel fillets and spoon the pan juices over and around.