Make sure your fish is thawed. You can put it in the refrigerator the night before. The morning you want to eat the fish, whisk together the miso, mirin, and sesame oil.
Using a paring knife make a few large hash marks in the fish so the marinade can penetrate. Swish the fillets in the marinade until fully coated. Cover and marinate in the refrigerator for at least 2 hours.
Heat a skillet over medium-high heat for 3-4 minutes. Add the neutral oil and the fish (it should lightly sizzle). If it’s sizzling intensely, reduce the heat.
Cook the fish for 4 minutes. Once you get a golden burnish on one side, flip, and reduce the heat to medium/medium-low if needed to ensure that the marinade doesn’t burn. A little bit of dark caramelization around the edges is okay, but the goal is to avoid burning the miso.
Cook for another 2 to 2.5 minutes. The middle should be just cooked and still a little juicy if you gently peek inside the fillet with a fork or chopstick. Serve with a grain and some veggies for an easy lunch or dinner!