Preheat the oven to 200°C.
Pour some hot (but not boiling) water over the spinach to loosen it up, then squeeze the excess water out. In a small bowl, mix it with the lemon zest, salt and pepper. You can add a pinch of cayenne or chilli flakes too, if liked.
Put the spinach in a baking dish then lay the cod fillets on top. Spray with the oil and squeeze the lemon juice over. Season to taste.
Cut the lemon into 2 slices and put them on top of the cod.
Cover the dish with foil then bake for 20-25 minutes in the oven. Don’t overcook the fish!
Serve warm.