In a small saucepan, bring the soy sauce, brown sugar, mirin, ginger and garlic to a boil over high heat. Lower the heat and simmer until the sauce is reduced by about half (6 tbsp. or so), about 15 minutes. Divide the sauce into 2 small bowls; reserve 1 bowl for serving.
Preheat a grill to high. Lightly oil the grate. Pat the salmon dry, brush with oil, then brush the flesh side with some of the sauce and season with pepper. Arrange the salmon on the grate skin side down, cover the grill and cook, occasionally basting with more of the teriyaki sauce, until the sides are just opaque, about 8 minutes.
Transfer the salmon to plates and drizzle with the reserved teriyaki sauce. Top with the scallion.