Q: How do I crisp up fish skin?
A: Start with the right fish.
Many larger fish—halibut, tuna, and cod, for example—have thick, leathery skin that is not very pleasant to eat. Salmon, char, snapper, and sea bass are better choices.
Next, learn what keeps skin from crisping. Liquid in any form—whether water or fat—is the enemy. So remove as much liquid as possible from the food both before and while you cook.
Before you heat the pan, press fillets firmly between paper towels to remove surface moisture. Once the fish is dry, preheat the skillet. High heat is necessary when the fish goes into the pan because cold skin will bond on a molecular level to a cool pan, making it impossibly stuck. But if you cook on high the whole time, you end up with blackened skin that is still flabby with fat and moisture underneath.
So start on high heat, and then drop it to medium-low to gently render fat. Lightly hold down the fish with a spatula so that the skin remains in contact with the pan for the first minute or so of cooking.
Cooking fish with the skin side down evaporates moisture, renders fat, and browns proteins. With steaks or burgers, you aim for even browning by flipping halfway through cooking. But with fish, you're better off cooking it almost all the way through skin side down. This maximizes crispness while providing a layer of insulation, allowing the delicate meat to cook gently. I cook my fish about 75% of the way through on the skin side, then flip (I know it's ready to flip when the skin lifts easily from the skillet). The result is shatteringly crisp skin, minimal fat, and moist, tender meat underneath.
Kenji Lopez-Alt is the chief creative officer of Serious Eats (seriouseats.com), where he writes The Food Lab, unraveling the science of home cooking.