Slice the cucumber into thin rounds, place them in a bowl, sprinkle with salt, and let sit for 5 minutes. Squeeze out excess water.

In a microwave-safe bowl, combine drained tuna with sake, soy sauce, and sugar. Mix well, cover lightly with plastic wrap, and microwave at 600W for 1 minute. Stir lightly after microwaving.

Mix white sesame seeds into the cooked rice. Add the cucumber slices and mix again. Serve the rice mixture in bowls.