In a small saucepan, bring soy sauce, brown sugar, mirin, ginger and garlic to a boil over high heat. Lower the heat and simmer until sauce is reduced by about half (6 tbs.) about 15 minutes. Divide sauce into 2 portions.
Brush salmon lightly with cooking oil, then brush flesh side with sauce and season with pepper. Grill, basting occasionally with sauce until sides are just opaque, about 8 minutes.
Serve with reserved sauce.
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