Before you start, bring a full kettle to the boil.
Combine rice and boiling water measure in a pot and bring to the boil with the lid on. Once boiling, reduce to lowest heat to cook for 12 minutes. Turn off heat and steam, covered, for 5 more minutes. Do not lift lid during cooking. Meanwhile, mix together vinegar, sugar and salt in small bowl until dissolved. Once rice is cooked gently stir through vinegar mixture and leave for a further 10 minutes with the lid on.
Add edamame to a heat-proof bowl and pour over boiling water. Leave to cook for 10 minutes before draining and season with salt. Set aside for later.
Prep radish, avocado and seaweed. Set aside for later.
Pat tuna dry and trim into neat loin if necessary. Season lightly with salt and sprinkle with sesame seeds, pressing down to ensure they stick.
Heat oil in a non-stick fry-pan on medium-high heat. Sear tuna for 30-40 seconds on each side, until lightly seared on all sides.
Thinly slice tuna.
Serve rice topped with tuna and garnishes. Serve sauce for dipping on the side.