Preheat the oven to 200˚C / fan 180˚C / gas 6. Heat a large non-stick, ovenproof frying pan until searing hot. Add the oil and then add the hake fillets, skin side down. Fry for 3 minutes and then pop the fillets on a lined baking tray in the oven for 5 minutes while you make the sauce.
To make the sauce, reduce the heat and add the butter to the same frying pan as used for the fish. Add the prawns and cook for 2 minutes. Add the wine, turn up the heat and bubble for 2 minutes. Add the capers and parsley and remove from heat. Season with freshly ground black pepper.
Meanwhile, place the spinach in a large pan with a splash of water. Add the nutmeg and heat until just wilted. Drain and season to taste. Divide between 4 warmed serving plates and top each with a hake fillet. Spoon over the prawn sauce and serve.