Mix vinegar, sugar, and salt in a microwave-safe container. Cover with plastic wrap and microwave at 600W for 30 seconds. Let cool.

Add the cooled vinegar mixture to warm rice in a bowl and mix gently to make sushi rice.

In a separate bowl, beat eggs and mix with sake, sugar, and salt.

Heat a lightly oiled tamagoyaki pan over medium heat. Pour 1/3 of the egg mixture, stir lightly, and fold when semi-cooked. Repeat with the remaining mixture. Cool and slice into 5mm strips.

Drain the oil from the tuna can. Mix tuna, mayonnaise, and salt/pepper in a bowl.

Prepare cucumber by cutting into quarters lengthwise. Cut seaweed into quarters. Slice tuna into 1cm thick strips.