Preheat your broiler and position the oven rack 4-6 inches from the heat source. Line a baking sheet with foil and lightly oil it.
In a small bowl, whisk together miso paste, mirin, sake, brown sugar, rice vinegar, and sesame oil until smooth and glossy.
Pat salmon fillets completely dry with paper towels and season lightly with salt. Place on the prepared baking sheet.
Brush half of the miso glaze over the salmon fillets, coating the tops evenly. Let marinate for 10 minutes at room temperature.
Broil the salmon for 6-8 minutes, then brush with the remaining glaze and continue broiling for 2-4 minutes more, until the glaze is caramelized and bubbly and the salmon flakes easily with a fork.
Remove from oven and immediately garnish with sesame seeds and sliced scallions. Serve hot with steamed rice or roasted vegetables.
Notes
Storage tip: Leftover salmon keeps in the refrigerator for up to 3 days. Reheat gently in a 275°F oven to avoid overcooking. The glaze can be made up to 3 days ahead and stored covered in the refrigerator.