In a large skillet, heat the butter and olive oil over medium heat.
Add the leeks and saute until they start to turn golden. Season the leeks to taste with salt and pepper.
Season the fish on both sides with salt and pepper while the leeks are cooking.
Add the wine or water and top the leeks with the seasoned cod filets.
Cover the skillet with a lid and reduce the heat to a simmer. Keep an eye on it to make sure the liquid doesn't evaporate, reduce the heat even more (and add more liquid) if necessary.
The fish is finished when the color turns opaque and the tip of a knife penetrates easily. The fish will flake, too. Start checking around 8 minutes for a 1/2 inch filet. It's better to under cook and have to return the fish to the heat than to overcook as the fish will get very tough.
Before serving, sprinkle the fish with a little paprika to add color and a little smoky flavor.