olive oil, for coating the aluminum foil
1/2 cup mirin
1/2 cup red miso paste
1/4 cup packed light brown sugar
1/4 cup soy sauce
2 tablespoons peeled and grated fresh ginger (from about a 2-inch piece)
1 wild salmon side (about 3 1/2 pounds), skin on and pin bones removed
2 medium scallions, thinly sliced (white and light green parts only)
Heat the oven to broil and arrange a rack in the middle.
Line a large rimmed baking sheet with foil and coat the foil with vegetable oil; set aside.
Whisk the mirin, miso, brown sugar, soy sauce, and ginger in a medium bowl until combined. Reserve 1/3 cup of the miso mixture in a small bowl; set aside.
Rinse the salmon under cold running water, pat dry with paper towels, and place on the baking sheet skin-side down. Using a sharp knife, score the salmon with parallel 1/4-inch-deep diagonal cuts spaced 2 inches apart.
Brush the salmon with all of the reserved 1/3 cup miso mixture.
(If the side of salmon is longer than the baking sheet, tuck the tail end of the fish under itself after the miso mixture has been applied.)
Cover with plastic wrap and refrigerate for 30 minutes.
Set a small fine-mesh strainer over a small bowl and strain the remaining miso mixture.
Discard the solids and set the strained sauce aside.
Remove the fish from the refrigerator and discard the plastic wrap.
Broil the salmon on the baking sheet, rotating once or twice, until it’s just opaque in the center and a golden brown crust has formed, about 10 to 12 minutes.
Remove the salmon from the oven, transfer it to a serving platter, and sprinkle it with the scallions and sesame seeds. Serve immediately, passing the strained sauce on the side.