How to Get Actually Crispy Fish Skin
Cooking fish is one of those things that looks easy, but can go awry so quickly. I can't tell you the number of times I got myself a nice skin-on filet for dinner only to have it turn out in a way that was so much less than I desired. One of the things that always got me was the skin part. In theory, fish skin is a perfect textural contrast to the tender flesh of the skin, crisping up to give the whole thing a flavorful texture balance. In actual execution, the skin can stick and tear away in sad ragged patches, or end up soggy.
In the culinary program I took, we spent several days on fish—fileting them, making stock from the bones, cooking them in parchment paper, and making them into very fancy fish sticks. But the application I've ended up using the most is the simplest one—cooking fish skin side down until it's almost done, flipping it to cook the other side for just a few seconds, and serving it with a simple salad. There are some tricks to getting the skin side perfectly crisp.
Let Your Fish Dry Out Before You Cook It
Moisture is the enemy of all things crispy. If your fish is wet when it hits the pan, it's going to steam rather than get that nice brown crispy crust. So before you do anything, you want to dry your fish really, really well. The best way to do that is first, take your fish out of the fridge and let it come to room temperature on a wire rack before you cook it. Then you can pat it dry with paper towels, or gently drag the blade of a chef's knife across the skin of the fish, a neat Thomas Keller trick I learned from Serious Eats. The liquid will collect on the blade of the knife and you can wipe it away. but again, be very gentle—fish is delicate and you don't want to destroy it before you eat it.
Don't Season Your Fish Until You're About to Cook
Seasoning your fish is important—that's where the flavor comes in. But if you add salt to your fish and let it sit a while before you cook it, it draws moisture to the surface of the fish, and remeber, moisture is no good here. Don't salt your fish until right before you put it in the pan.
Use the Right Pan
It's tempting to use a nonstick pan for cooking fish—after all, you don't want the skin to stick, and it has nonstick right in the name. But a nonsticks pan won't give you as nice and even a sear on the fish skin as a stainless steel or cast iron pan. Though I love my cast-iron pan, I prefer to use stainless steel here, but if you have a very well seasoned cast-iron pan, go for it.
A Dusting of Flour or Cornstarch
If you really want some insurance for getting things crispy, you can always sprinkle a small amount of flour or cornstarch on the skin side of the fish. This is sort of an old school French technique that's gone out of favor, and no worries if you don't want to use flour in your cooking for whatever reason. But it definitely works—just make sure you brush off any excess so that the flour doesn't burn.
Press the Fish in the Pan
Once you get your pan nice and hot, and then add oil, and then get that oil nice and hot, you can add the fish in there skin side down. (A good way to tell if it's hot enough is to gently drag the fish across the pan before you drop it in—if it sticks wait for it to heat a bit more.) Fish is naturally going to curl up once you put it in there, so to ensure that your skins gets in contact with the pan evenly, you're going to want to use a fish spatula to gently press the fish so the skin side maintains contact with the hot pan.
Know That Skin Will Release When It's Ready
When you first put the fish in the pan and start pressing with your spatula, the skin might stick. But don't worry—as long as you've got enough oil in there and the heat is high enough, the fish will release from the bottom of the pan after a minute or two. Be patient and don't panic, and you'll get yourself a perfectly cooked piece of fish with very crisp, delicious skin.