Preheat the oven to 180 ºC for about 15 minutes. Cut the potatoes into 1 cm thick slices and arrange on the bottom of an oven dish. Julienne the onions and arrange on top of the sliced potatoes (make sure the onions and potatoes are roughly the same thickness so that they will take the same time to cook.)
Drizzle some extra-virgin olive oil generously over the top to cover the potatoes and onions and add salt and pepper. Bake for 10 minutes without opening the oven door.
Open the oven door, add another dash of extra-virgin olive oil and pour the white wine evenly over the potatoes and onions. Add the cod steaks with the skin facing downwards.
Bake for another 20 minutes or so (depending on the oven) and you’re good to go!