Prepare the mahi mahi fillets: Pat the filets dry on both sides with using a few sheets of paper towels.
Lightly brush the fillets with olive oil on both sides. Set aside.
Make the blackened seasoning by mixing together the smoked paprika, onion powder, garlic powder, dried oregano, dried thyme, ground black pepper, and Kosher salt in a shallow baking dish or large plate. Make sure that it is fully blended.
Dredge each mahi mahi filet in the blackened seasoning, making sure that they are coated on both sides. Set aside.
To blacken mahi mahi on the stove, start by heating a cast iron skillet (or a frying pan) over medium-high heat.
Add the butter and cook until fully melted but not browned.
Gently add the seasoned mahi mahi fillets to the pan. Do not overcrowd the pan. If all four of them don’t fit, you can do it in two batches.
Cook for 2 minutes, then gently flip and continue to cook the other side for an additional 2 minutes. To test for doneness, try to flake a small area of the fish. It should flake easily and look opaque. If it does not, cook for an additional 30 seconds per side until done. If you have a digital thermometer, you can insert it into the thickest part of the fish. If it registers 130 degrees F, it should be cooked.
Remove from the skillet, cover it with aluminum foil, and let your blackened mahi mahi rest for a few minutes. Serve with lime or lemon wedges on the side. If preferred, garnish with mango salsa.