Heat the oven to 220C/fan 200C/gas7. Wilt the spinach with a little butter in a pan or in the microwave and season well. Press out any excess liquid with the back of a spoon then divide between two individual ovenproof gratin dishes and sit the fish on top.
Mix the crème fraîche, mustard and cheese then season. Spread over the top of the fish then sprinkle on some breadcrumbs. Bake for 15-20 minutes until golden, bubbling and fish is cooked through.