For the sesame green beans, finely grind 50g/1¾oz sesame seeds in a spice grinder or a mortar. Transfer to a food processor or blender. Add the remaining sesame seeds, soy sauce and sugar and process into a paste. Set aside.
Bring a saucepan of salted water to a boil. Add the beans and cook for 4 minutes, or until tender but still crisp. Using a slotted spoon, transfer the beans to a large bowl of iced water and leave to chill for 10 minutes. Drain and dry the beans. Whisk the vinegar, spring onions and ginger with the sesame seed paste in a bowl. Add the chilled beans and toss in the paste mixture until thoroughly coated.
To prepare the dressing, place all of the ingredients in a small lidded jar. Firmly seal and shake until completely combined. Taste and season with salt and pepper, if desired.
Place the tuna and avocado on a serving plate and drizzle the dressing over the top. Dress with the rocket leaves and coriander and sprinkle with the toasted sesame seeds. Serve the salad with the green beans alongside.