Preheat the oven to 200 °C (gas mark 3 to 4).
Layer half of the potato slices and sage in an ovenproof dish.
Drizzle with a little olive oil, then evenly distribute the fish fillets on top and sprinkle with half of the Parmesan.
Cover with the remaining potatoes, add the remaining sage leaves and Parmesan, salt and pepper, and drizzle the oil over the top. Put the dish in the oven for 25 to 30 minutes.