For cooks who love fish, commercially quick-frozen portions can be a tremendous convenience. Unlike fish bought fresh, frozen fish has a shelf life that's measured in months. This makes it possible to have fish any time, anywhere in the country, whether it's in season or out. The only down side is that thawing the frozen portions requires advance planning. Fortunately, most frozen fish can be readily cooked from frozen.
Fresh vs. Frozen
Many shoppers assume that fresh fish is automatically superior, and that buying and serving frozen fish is in some way a compromise. In reality, unless you've caught the fish yourself or bought it straight from the boat, "fresh" fish is usually a few days old by the time it reaches your shopping cart. In contrast, the fish in your shop's frozen section are usually flash-frozen immediately after they're harvested and processed. This extremely rapid freezing -- usually done at the fish plant or even on the fishing boat -- preserves the fresh flavor and texture of the fish at a very high level of quality.
Thawing vs. Cooking from Frozen
Most recipes assume you're working with fresh fish, so to use them without adjustment, most cooks begin by thawing their portions. That returns the fish to like-fresh condition, and it will cook almost identically to a similar portion of fresh-cut fish. However, this requires time, and if the fish isn't thawed properly, it can increase your risk of foodborne illness. Cooking from frozen eliminates both the inconvenience of defrosting and the risk of illness, two distinct benefits. Cooking from frozen also makes it harder to overcook fish, one of the most common mistakes in preparation.
Most normal cooking techniques require little alteration if you're working from frozen, other than slightly extending the cooking time. This holds true for poached or steamed fish, and for fish that's broiled or baked in your oven. Pan-fried fish should be covered for the latter half of its cooking time, once it's thawed enough to begin releasing juices. The steam will help cook it in the middle. Thick portions can benefit from being wrapped in a foil or parchment pouch, which also traps the steam and helps cook the fish. Clamshell-type contact grills work well with thin fillets, which cook quickly but remain moist if they're started from frozen.
A Few Tips
Most frozen fillets and whole smaller fish are individually quick frozen, so you can remove just one or two pieces as needed. Fish that are frozen together in a solid brick aren't suitable for cooking from frozen, and should be thawed and separated first. If the fish is labeled as individually frozen but the pieces have clumped together, that's a sign that it was thawed and re-frozen. Don't buy it, and be vigilant about purchasing from that supplier in future. Cooking from frozen works best with thin portions, so whole larger fish or fillets thicker than an inch are usually better if thawed first.