Preheat the oven to 180C/350F/Gas mark 4. Place the Cajun seasoning on a flat plate and use to dust the hake fillets, shaking off any excess. Heat an ovenproof frying pan. Add the olive oil and sear the hake skin side down for 1 minute, then turn over and cook for another minute. Transfer to the oven and roast for 4-6 minutes. The exact time will depend on the thickness of the fillet.
Meanwhile, wash the spinach and remove any large stalks, then dry well. Heat the butter in a large pan and then add the spinach, cook until it wilts down, season to taste. Arrange on warmed plates (leaving any excess moisture behind in the pan) and top each one with a piece of blackened Cajun hake. Garnish with a lemon wedge to serve.