1. Combine first 7 ingredients in a small bowl. Pour 2 tablespoons juice mixture into a zip-top plastic bag; reserve remaining juice mixture. Add tuna to bag; seal and marinate at room temperature 20 minutes, turning bag once.
2. Cook noodles according to package directions. Drain and rinse with cold water; drain. Combine noodles, carrots, vinegar, sesame seeds, sesame oil, and mirin; toss well.
3. Heat a grill pan over medium-high heat. Coat pan with cooking spray. Remove tuna from bag; discard marinade. Place tuna in pan; cook 1 1/2 minutes on each side or until desired degree of doneness. Divide noodle mixture between 2 plates; sprinkle each with 1 1/2 teaspoons onions. Cut tuna into thin slices; arrange 1 sliced tuna steak on each plate. Drizzle each serving with 3 tablespoons reserved dressing.