How to Make It
In a small skillet, toast the sesame seeds over moderate heat until golden, about 3 minutes; transfer the seeds to a spice grinder and let cool. Finely grind the seeds and transfer them to a bowl. Whisk in the mayonnaise, miso and dry sake, then whisk in the water.
In a large bowl, whisk the ume vinegar with the lemon juice, sesame oil and mirin. Gradually whisk in the canola oil and season with salt. Add the tomato wedges and sliced scallions and toss. Spoon the tomato salad with its juices onto a large platter and drizzle with the sesame-miso sauce. Garnish the salad with the crumbled nori and serve immediately.