First publicized by the Canadian Department of Marine Fisheries, this method is tried and true.  We found this easy step by step on http://www.ehow.com/how_4804619_long-cook-fish.html.   The site has lots of great seafood cooking tips, recipes and how-to videos.

This technique works with the following cooking methods: broiling, grilling, poaching, steaming, sautéing, microwaving, en papillotte, planking, and baking (the oven should be between 400 and 450 degrees Fahrenheit).

Measure your portion of fish at its thickest part, and calculate 10 minutes of cooking time for each inch of fish. Don’t forget to add or subtract fractions of inches—no rounding up or down.

Fish will continue to cook after removed from heat, so visually checking your portion of fish and taking it out when it “looks” done will leave you with an overcooked piece of fish.

If your piece of fish is more than ½ inch thick, you will need to divide the cooking time in half, and turn the fish halfway through cooking.