Season the tuna steaks with the crushed peppercorns. Heat a griddle pan over a medium-high heat and griddle the tuna steaks for 2 minutes on each side until browned but still pink in the centre. Remove from the pan and leave to rest.
Put the sugar snap peas in a steamer basket and lower into a shallow saucepan of boiling water so that the peas are not quite touching the water. Drizzle with the sesame oil, cover and steam for 2–3 minutes or until tender. Alternatively, cook the peas in a bamboo or electric steamer.
Place all of the dressing ingredients in a screw-top jar and seal with a tight-fitting lid. Shake vigorously until well combined.
Divide the sugar snap peas between 4 serving dishes, then cut the tuna steaks in half diagonally and arrange over the peas. Drizzle with the prepared dressing and sprinkle with the sesame seeds. Serve immediately, with cellophane rice noodles, if liked.