This is one of my favorite ways to prepare salmon. The miso gives it such a wonderful flavor that can't be beat. The recipe makes a lot (enough for 8 people), but you can use the leftovers (if there are any) for many things. Try flaking some of the salmon over a tossed garden salad for a little extra flavor.
The miso-ginger glaze can be made up to 1 day in advanced if kept in the refrigerator in an airtight container.
Arrange oven rack in the middle, and preheat the oven to broil. 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 the top with the scallion slices and sesame seeds. Serve immediately, passing the strained sauce on the side.