Blackened mahi mahi is a spicy preparation, but it's not one of those too-painful-to-enjoy experiences. While blackening traditionally calls for a high-heat sear, the most important component is a liberal application of blackening or Cajun seasoning mix. This particularly flavorful approach to mahi mahi complements its mild taste and hearty texture well. Mahi mahi does well in a high-heat oven without drying out, as long as you keep an eye on the clock.
Defrost frozen mahi mahi so it cooks efficiently and evenly in the oven. Put it in the refrigerator for 12 to 24 hours. Otherwise, submerse it in airtight packaging in cold water for about an hour, switching out the water for colder water halfway through.
Preheat the oven to 450 degrees Fahreheit for at least 20 minutes. High heat helps crisp the outside of the mahi mahi and give it a blackened appearance; you're not burning the fish, though -- just browning it and its coating of seasoning well. Cover a baking tray with a sheet of foil and coat it with nonstick spray.
Coat the top of the mahi mahi filet or steak with a good dousing of blackening or Cajun seasoning mix. If you don't have a pre-made mixture, make your own. Paprika is the primary base ingredient. Combine it with a little less onion powder and garlic powder. Then, mix in ground black and white pepper and a few spicy peppers, like cayenne, chipotle and chile pepper powders. Add a bit of salt, thyme, oregano or other complementary herbs and spices to taste.
Put the mahi mahi in the top third of the oven. Bake the steak or filet for approximately 10 minutes per inch of thickness. Cook it to 145 F at center, as confirmed with an instant-read food thermometer. If you don't have one, the fish is safely cooked once its flesh becomes uniformly opaque all the way through; just cut into the middle to check.