Step 1

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

Step 2

Meanwhile, place scallions in a medium heatproof bowl. Combine butter, sesame oil, ginger, garlic, and remaining 4 Tbsp. 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 1 minute, then season with salt.

Step 3

Serve fish with scallion-sesame butter spooned over.