Marinate the salmon by first putting the fillets in a shallow dish large enough to hold them in a single layer.
In a small mixing bowl, stir together the mirin, miso, sugar, soy sauce, garlic, ginger, and sesame oil.
Pour the marinade over the salmon fillets, turning them to coat on both sides. Cover the dish with plastic wrap and marinate in the refrigerator for at least 3 hours or as long as overnight
About half an hour before serving time, preheat the broiler.
Transfer the salmon fillets to a broiler pan or baking dish large enough to hold them in a single layer. Place them under the broiler and cook until nicely browned and barely cooked through in the center, 3 to 4 minutes per side (if the broiler is on high) or 6-7 minutes per side (if the broiler is on low) .. depending on thickness, turning them carefully with a spatula half way through.
Serve as a whole fillet with your favorite vegetable side .. or shred & incorporate as a protein source into anything from omelets to salads.