Dry the fish with paper towels and season it with Tajin. Dust one side with a uniform layer of cornstarch, then remove any excess. Heat 2 tablespoons of olive oil in a large nonstick skillet over medium-high heat. Place the fish in the skillet, cornstarch side down, and cook, shaking the pan occasionally, until the edges are golden, crisp, and opaque, about 6–8 minutes. Flip the fish and cook for an additional 1–2 minutes until the fillets are cooked through and easily flake with a fork.

Meanwhile, put the scallions in a medium heatproof bowl. In a medium saucepan over medium heat, combine the butter, sesame oil, ginger, garlic, and 4 tablespoons of olive oil. Cook until the butter starts to foam and the garlic turns light golden, approximately 4 minutes. Pour this mixture over the scallions, stirring until wilted. Allow it to cool for 1 minute, then season with salt.

Serve the fish with the scallion-sesame butter spooned on top.