Zest and juice the limes, peel and grate the ginger, mix these together with the maple syrup and soy sauce, coat the fish fillets in it and leave in the fridge for an hour.
Peel and grate the beetroot and potatoes. Put them into a bowl. Boil a kettle of water and pour over the beetroot and potatoes, leave for a couple of minutes to soften the vegetables. Drain the water of and dry the mix well. Season with the salt and pepper and form into balls about the size of tennis balls, flatten them to a thickness of 1 centimetre.
Heat 2 large frying pans on a medium heat and split the oil between them, when the oil is hot add the rosti and cook for 2 minutes on each side, they should be crispy and golden.
Add the marinated fish to the other pan and cook skin side down for 4 minutes or until the skin goes crispy. Turn over and cook for a further minute. If the fillets are thick you may need to cook for a little longer.
Serve the fish on top of the rosti and spoon some of the cooking juices over the top.
«You can use cod or haddock in place of the black cod.»