To cook fish well, all you need to know is that most fish are best treated simply. The Canadian Cooking Theory, a timing technique popularized by the late cooking authority James Beard, advises cooking fish for 10 minutes per inch of thickness regardless of cooking method, cut, or species of fish. This is about as good as generalizations get. But if you follow it religiously, your fish will often be overcooked and occasionally undercooked. (You would never cook a sirloin steak by the clock, would you?) As a general guideline, 8 to 9 minutes per inch of thickness works a little better than 10ΓÇöbut only as a guideline.
Timing gives an approximate sense of when a fish is likely to be done, but you have to check it all the while. Even under the best of circumstances, cooking is inexact. And many things can affect the speed at which fish cooks: The actual temperature of the oven, grill, or skillet; the shape, thickness, density, and temperature of the flesh; and the presence or absence of liquids, for example.
Almost all recipesΓÇöincluding those hereΓÇögive some timing guidelines for knowing when fish is done. But you will soon learn to recognize the signs of doneness: a firming up of texture; the beginnings of flakiness; an opaque, whiter look throughout.
In addition, there are two surefire ways to determine whether fish is done: interior appearance and interior temperature. When a fish is opaque throughout, it is done. When you begin to suspect that a piece of fish has nearly finished cooking, take a thin-bladed knife and gently prod between the flakes of fillets or steaks, or cut between flesh and bone. Do not be afraid to poke around while fish is cooking; if you like your salmon or swordfish just underdone or your tuna medium-rare, learn what it looks like inside. Remove the fish just before it reaches the stage at which you want to eat it; it will finish cooking between kitchen and plate.
Just as accurate is an instant-read thermometer. Insert it into the thickest part of the fish. All fish is cooked through at 137┬░F. Usually, 135┬░F leaves just a hint of translucence and more moisture and is done enough for most people. For tuna and other fish that you might prefer less well-done, try 120┬░F for starters. Use the knife-peeking technique to double-check the thermometer if you are unsure.
