The classic time guide for cooking fish is to cook it for ten
minutes for every one inch of thickness. But this really isn't a
satisfactory method as it leaves too many questions open - At what
oven or stove top temperature? What kind of fish? Oily or dry
texture?
And, as Molly Stevens at FineCooking writes:
"The often-quoted theory of cooking fish for 10 minutes per
inch of thickness may be a good guideline, but in reality 8 minutes
is a better timeframe in which to at least start checking for
doneness. Fish will continue to cook for a
minute or two off the heat. Be sure to stop cooking when the fish
is just shy of done; otherwise, it will overcook by the time you
serve it."
But that begs the question - how do you check it for doneness?
Here are a couple of alternatives:
A quick video lesson from Food52 on How to Know
When Fish is Done teaches us that:
1) The fish should be opaque in color on the outside, and, if
the fish is thick, also on the inside.
And here's their test for the courageous among us:
2) Take a small metal skewer or thin knife and insert it into
the middle of the fish for five seconds. Then carefully put the
skewer against your bottom lip. If the skewer is cold, the fish is
undercookeed; if it's warm, the fish is just right; if it's hot,
it's overcooked. Of course, if the latter, you run the risk of a
burned lip.
However, we're not really that courageous. So we just take
the fish's temperature with an instant read thermometer (which
actually makes a great metal skewer if you want to doublecheck and
use the method above):
First try to ensure the fish has an even thickness by
folding under any especially thin end pieces. Then cook the fish to
137 degrees and let it rest - it should go up to 140 degrees.
If you're dealing with tuna or swordfish steaks, and want
them to be medium-rate, shoot for 125 degrees.
Finally, we should note that the government does recommend
145 degrees for fish fillets, so if you have any safety concerns
that is the safest temperature, although the fish may be a little
dry.