In a small bowl, combine the onion powder, garlic powder, cayenne pepper, oregano, salt, and paprika.
Pat the mahi mahi fillets dry with paper towels. Brush both sides with 1 tablespoon of the avocado oil.
Evenly coat both sides of each fillet with the seasoning mixture, using about ½ teaspoon per fillet.
Heat the remaining 1 tablespoon of avocado oil in a large cast iron skillet over medium-high to high heat until hot.
Add the fillets to the skillet and cook undisturbed for 2 minutes to develop a blackened crust.
Flip the fillets and cook for an additional 2–3 minutes, or until the fish reaches an internal temperature of 145°F and flakes easily with a fork.
Remove the fillets from the skillet and let them rest for 5 minutes before serving.
Notes
Refrigerate the leftovers in an airtight container for up to 3 days. Use an instant-read meat thermometer to make sure that the mahi mahi is cooked to a safe temperature before removing it from the grill. You can use any oil with a high smoke point for this recipe instead of avocado oil. Vegetable oil, Canola oil and Coconut oil would all work great for this recipe.