Thaw salmon, if frozen. Place salmon fillets on greased unheated rack of broiler pan. Measure thickness of salmon. Brush salmon with 1 tablespoon of the balsamic vinegar; sprinkle salmon with salt and pepper. Broil salmon 4 to 5 inches from heat for 4 to 6 minutes per 1/2-inch thickness or until salmon flakes easily with a fork.
Meanwhile, in bowl whisk together remaining vinegar and pesto until combined. Remove 2 tablespoons pesto mixture. Add cabbage to pesto mixture in bowl; toss to coat.
To serve, place cabbage mixture on plate; top with salmon. Drizzle reserved pesto mixture. Sprinkle with green onion slices. Makes 4 servings.