Start this recipe 2 days ahead.
In a bowl, mix Saikyo miso, sake, mirin, and salt until well combined.
Place the salmon fillets in a resealable plastic bag or shallow dish and pour the marinade over them, ensuring each fillet is well-coated. Refrigerate for 2 days (48 hours).
Preheat your grill (broiler) to (190°C). Line a baking sheet with parchment paper.
Remove the salmon from the marinade and gently wipe off any excess with a paper towel.
Place the salmon on the prepared baking sheet and bake for 6 - 7 minutes (skin side up, then flip and grill for an additional minute. Salmon should be just cooked through and skin slightly caramelized.
Optional: Garnish with lemon wedges and sliced green onions.