Shred the imitation crab sticks into thin strands using your fingers.
Julienne the cucumber and carrots; pat the cucumber dry to remove moisture.
Whisk mayo, sriracha, rice vinegar, sugar, sesame oil, and chili flakes in a small bowl.
Add kani, cucumber, carrots, and green onions to a mixing bowl.
Pour the dressing over and toss gently until evenly coated.
Sprinkle sesame seeds and optional masago before serving.
Notes
Use Japanese mayo for the most authentic flavor.Chill vegetables before assembling for extra crunch.Add mango for a sweet contrast to the spicy dressing.Mix dressing into the salad right before serving for best texture.Real crab can be used instead of imitation crab.