How to Make It
Whisk together soy sauce, vinegar, ginger, and sugar in a small bowl. Measure 2 tablespoons soy-ginger sauce into a separate small bowl for brushing salmon; reserve remaining sauce for serving. Brush salmon all over with 2 tablespoons sauce; sprinkle evenly with salt. Chill 15 minutes.
Preheat oven to broil with rack 6 to 8 inches from heat source. Arrange salmon, skin sides up, on a large rimmed baking sheet. Broil in preheated oven until skin is lightly charred, about 5 minutes. Flip salmon; broil until salmon flesh just flakes with a fork, about 3 minutes.
Place salmon collars on plates. Toss together radishes and cabbage in a bowl; divide among plates with salmon. Drizzle with some of the reserved soy-ginger sauce. Serve with lime quarters.