Look for Japanese soba noodles in the ethnic-food section of your supermarket.
Cook noodles according to package directions; drain and rinse under cold water. Drain; set aside.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Sprinkle both sides of tuna with 1/4 teaspoon salt and black pepper. Place tuna in pan, and cook for 3 minutes on each side or until desired degree of doneness. Transfer to a platter; cool slightly. Cut tuna into 6 equal pieces.
Combine noodles, remaining 1/4 teaspoon salt, cucumber, and remaining ingredients except sesame seeds in a large bowl; toss well to combine. Arrange 1 cup noodle mixture onto each of 6 plates. Top each serving with 1 teaspoon sesame seeds and 1 tuna piece.