How to Make It
Put the eggs in a saucepan with cold water and bring to a boil. Cover and let stand off the heat for 10 minutes. Drain and shake the eggs to crack the shells. Fill the pot with cold water and ice; let cool. Peel the eggs and transfer to a plastic bag. Add the mirin and soy sauce and refrigerate for 30 minutes or overnight.
Bring a large pot of salted water to a boil and fill a bowl with ice water. Boil the green beans until crisp-tender, 3 minutes. Using a slotted spoon, transfer the beans to the ice water, then drain and pat dry. Add the potatoes to the boiling water and cook until tender, about 12 minutes. Drain and add the potatoes to the ice water to cool slightly. Peel the potatoes and slice 1/3 inch thick.
Preheat a grill pan. Season the tuna with salt; rub with the sesame oil. Grill the tuna over high heat for 3 minutes, turning once, for medium-rare. Let stand for 5 minutes, then slice the tuna.
In a mini food processor, combine the miso, vinegar and mayonnaise. Slowly add the vegetable oil and process the dressing until creamy.
Drain the eggs and halve them. On a large platter, arrange the greens, vegetables, eggs and tuna. Sprinkle with sesame seeds. Pass the nori sheets and dressing at the table.