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Mix the flour, polenta, orange zest and thyme leaves together with some seasoning. Dip the fillets in the beaten egg then roll in the polenta mix to coat. The fish can now be chilled for a few hours before you're ready to cook.
Heat the butter and olive oil in a large frying pan until foaming, add the fillets and lower the heat. Cook for 3 mins on each side. Meanwhile, wilt the spinach in a large pan, drain off the excess water, season and divide between 4 plates. Serve with the fish fillet on top.