Mix the flour, paprika, oregano, thyme, garlic powder, onion powder, cayenne pepper, salt, and black pepper in a shallow dish. This is your blackening seasoning mix.
Pat the tuna steaks completely dry with paper towels. This step helps the seasoning adhere and promotes a good sear.
Dredge each tuna steak in the flour mixture, pressing gently to coat all sides evenly. Shake off any excess flour.
Place a large cast iron skillet over medium-high heat. Add the olive oil and 1 tablespoon of butter. Heat until the butter foams and begins to brown slightly.
Carefully place the seasoned tuna steaks in the hot skillet. Do not overcrowd the pan; cook in batches if necessary.
Sear the tuna for 2 to 3 minutes per side for a medium-rare interior. The exterior should develop a dark, blackened crust.
Remove the tuna from the skillet and set aside on a cutting board. Cover loosely with foil and let rest for 5 minutes.
While the tuna rests, prepare the sauce. Reduce the heat to low. Add the remaining 2 tablespoons of butter and the minced garlic to the same skillet. Cook for 30 seconds until fragrant.
Remove the skillet from the heat and stir in the lemon juice. Spoon the warm lemon-butter sauce over the rested tuna steaks before serving.
Notes
For the best results when searing cast iron tuna, ensure your skillet is very hot before adding the fish.
If you prefer your tuna cooked more than medium-rare, reduce the heat slightly after the initial sear and cook for an additional minute per side.
You can substitute the cayenne pepper with a pinch of crushed red pepper flakes for a different type of heat profile.