Combine all marinade ingredients in a medium bowl (miso, mirin, brown sugar, and soy sauce) until the brown sugar has completely dissolved. Set aside 1 tablespoon of the marinade to use as a sauce later.
Place salmon in the marinade bowl, and flip to coat well. Place skin side up, and spoon extra marinade on top.
Cover with plastic wrap and place in the fridge to marinade for 1-2 hours.
Preheat your oven broiler to high, and place the top rack of your over 8" away from the bottom of your broiler. Place the marinated salmon skin side down on a baking sheet lined with foil (for easy clean up), making sure to leave a few inches between the filets. Broil until the salmon is golden on top and flakey in the center, about 8-10 minutes. This will vary with how thick your salmon is, for thinner salmon, check around 6 minutes. Thicker filets can go longer. I prefer to pull the salmon while the center is a bit under, and allow it to come to temp on the counter as it rests (way better than over cooked salmon!).
Sprinkle salmon with sesame seeds and green onions before serving, and enjoy!