Rinse rice under cold water and place in a lidded saucepan.
Add 568ml of water and simmer for 25 minutes until water is absorbed.
Remove from heat and let the rice stand covered for 10 minutes.
Spread rice on a tray and mix with mirin while fanning to cool.
Place a seaweed sheet on a bamboo mat and moisten fingers with water.
Spread a 0.5cm layer of rice over three quarters of the seaweed.
Apply wasabi, tuna, spring onion, and cucumber in a line near the center.
Roll the mat tightly and let sit with the seam side down.
Trim ends with a wet knife and cut each roll into 6 to 8 pieces.
Serve with soy sauce and pickled ginger.