Pat fish dry with paper towels, season with Tajin. Sprinkle one side with an even layer of cornstarch; brush off excess. Heat 2 tablespoons olive oil in a large nonstick skillet over medium-high. Lay the fish and cornstarch side down in a skillet and cook, shaking the pan occasionally, until golden, crisp, and opaque around the edges, 6–8 minutes. Turn fish over and cook until fillets are cooked and flake easily with a fork, 1–2 minutes.

Meanwhile, place the scallions in a medium heatproof bowl. Combine butter, sesame oil, ginger, garlic, and 4 tablespoons of olive oil in a medium saucepan over medium heat. Cook until butter begins to foam and garlic turns light golden, about 4 minutes. Pour over scallions, stirring to wilt. Let cool for 1 minute, then season with salt

Serve fish with scallion-sesame butter spooned over