Preheat the oven to Gas mark 4, 180ºC (350ºF). Place the cajun seasoning on a flat plate and use to dust the haddock fillets, shaking off any excess.
Heat an ovenproof frying pan. Add the olive oil and cook fish, skin side down for 1 minute, then turn over and cook for another minute. Transfer the pan to the oven and roast for 4-6 minutes. The exact time will depend on the thickness of the fillets.
Meanwhile, heat the butter in a large pan and then add the spinach. Cook until it wilts down, season to taste.
Arrange the spinach on warmed plates and top each one with a piece of haddock. Serve with a lemon wedge and crusty bread.