Place washed rice in a rice cooker, add water slightly below the 2-cup line, and cook as usual.

Slice green onions into small pieces, julienne shiso leaves, and ginger.

Thinly slice the tuna.

Mix mirin and soy sauce in a bowl, microwave uncovered for 30 seconds to 1 minute at 600W, and let cool.

Add tuna slices to the marinade, cover with plastic wrap, and refrigerate for 10 minutes.

Mix sugar, salt, and vinegar in a separate bowl to make sushi vinegar.

Combine cooked rice with sushi vinegar, mix gently, then add ginger, shiso leaves, sesame seeds, and 1 tbsp of the tuna marinade.