Heat the olive oil in a large pan on a medium heat. Add the chopped leeks, season with salt and pepper and cover with a lid. Cook for 3 minutes, then add the cream.
Reduce the heat to low and cook for a further 4 to 5 minutes.
In the meantime, cook the cod in a steamer or in a pan
of simmering salted water for 12 minutes.
Melt the butter in a second pan set over a high heat. Add the apple slices and cook for 3 to 4 minutes, until lightly caramelised.
Serve the fish on a bed of creamed leeks and caramelised apples and drizzle with the pan juices.