- In a medium bowl, whisk together egg yolk, lemon juice, mustard, salt, and water until frothy.
- Slowly drizzle about ¼ cup oil into egg mixture, whisking constantly, until mayonnaise is thick and oil is incorporated. When mayonnaise emulsifies and starts to thicken, add remaining oil in steady, thin stream while whisking.
- Fold in ginger, sesame oil, and lime zest.
- In large mixing bowl, combine tuna with 4 tablespoons prepared mayonnaise (or more if desired) and season to taste with salt and black pepper.
- Fold in cucumber, scallions, bell pepper, and lime juice. Refrigerate until ready to serve.