1. Combine sake, 2 tablespoons rice wine vinegar, 2 tablespoon soy sauce, 1 tablespoon oil, 1 tablespoon miso, and the black pepper in a small bowl.
2. Arrange salmon fillets in a single layer in a glass baking dish. Pour marinade over salmon and cover dish with plastic wrap. Marinate in the refrigerator 2 to 24 hours, turning fillets once.
3. Heat grill over medium heat. Oil cooking grate. Remove fillets from marinade. Reserve marinade. Brush skin side of fillets with 1 tablespoon oil. Arrange fillets, skin side down, on grill; top each with 2 sprigs of tarragon. Brush salmon with reserved marinade. Close lid and grill fillets 8 to 10 minutes, or just until cooked through.
4. Meanwhile, whisk together 2 tablespoon vinegar, 4 teaspoons soy sauce, the sugar, 1/2 teaspoon miso and 1 tablespoon oil in a small bowl.
5. Transfer salmon to a large serving plate. Drizzle dressing evenly over the fish and garnish with cucumber. Makes 6 servings.