Cut carrots, celery, and cucumbers into thin 2-inch-long strips and place in a bowl; set aside.
Stir together rice vinegar, 1 teaspoon oil, and next 3 ingredients in a small bowl. Measure 1/2 cup vinaigrette mixture and toss with vegetables.
Brush tuna with remaining oil; sprinkle with salt and pepper. Place a large nonstick skillet over medium-high heat until hot. Add tuna, and cook 3 minutes on each side or until desired degree of doneness. Serve with vegetable slaw and drizzle with remaining vinaigrette mixture. Garnish, if desired.