All great fish dishes begin with a piece of good-quality fish, which can be purchased either fresh or frozen. "Frozen," it should be noted, does not mean that the fish is not fresh. In fact, much of the fish we buy has been frozen at some point, whether it's found in the freezer section or displayed on ice at the fish market.
For fillets, the best way to determine freshness is by smell. A fillet should be virtually odorless; a fishy smell indicates that a large number of bacteria have already developed. It should also be free of bones, bits of skin, and blood (not necessarily a sign of freshness, but of a good fishmonger), and should not curl at the edges or show signs of yellowing.
Determining the freshness of a whole fish is slightly easier. Again, the fish should not give off an odor, and its eyes should be clear, not cloudy. The skin should be shiny and adhere tightly to the flesh.
After purchasing, rewrap fresh fillets tightly in plastic, and place them in the coldest part of the refrigerator, preferably on ice; cook them as soon as possible. Defrost frozen fish overnight in the coldest part of the refrigerator (a thick fillet or whole fish may take up to two days to defrost). Slow, cold defrosting discourages "drip loss," so the cooked fish will be moist and the texture firm, but not tough.