Julienne the carrot, sprinkle with salt, massage it, and let it sit for 10 minutes. Squeeze out the excess water.

In a microwave-safe container, mix the seasoning ingredients (sake, sugar, soy sauce, mirin) and heat for 30 seconds at 600W. Add the tuna and mix well.

Boil the eggs in a pot with enough water to cover them for 12 minutes. Cool under running water and peel the shells.

Spread a thin layer of mayonnaise on one side of each slice of bread.

Place one slice of bread (mayonnaise side up) on a sheet of plastic wrap. Layer the carrot, boiled eggs, tuna mixture, and lettuce in order. Top with the second slice of bread (mayonnaise side down). Wrap tightly with the plastic wrap and let it sit for 10 minutes.