Combine the tuna, raisins, scallions, mayo, fish sauce, and yuzu juice in a medium bowl. Season with salt and pepper.

To set up a sandwich, place the 4 slices of bread on a cutting board. Top two of the slices with 3 pieces of butter lettuce.

Split the tuna salad on top of the butter lettuce covered bread and spread into an even layer. Top each with the remaining slice of bread. Slice on a diagonal and serve immediately.