To prepare rice: Bring 1 1/2 cups water to a boil in a small saucepan over medium heat. Stir in rice, reduce heat to low, cover, and simmer at the lowest bubble until the rice is tender, about 45 minutes. Remove from heat and let stand, covered, for 10 minutes.

Meanwhile, to prepare tomatoes: Bring a large saucepan of water to a boil. Place a medium bowl of ice water next to the stove. Using a sharp paring knife, core the tomatoes and score a small X through the skin on the bottom of each.

Add the tomatoes to the boiling water, in batches, and cook until the skins are slightly loosened, about 2 minutes. Using a slotted spoon, transfer the tomatoes to the ice water and let stand for 1 minute. Peel with a paring knife. Quarter the tomatoes lengthwise and scoop out the seeds.

Return the water to a boil. Add kombu, tamari, scallions and ginger and cook, stirring for 5 minutes. Remove from heat and add the tomato slices. Let marinate while the rice cooks, about 20 minutes. Remove the tomatoes from the marinade and gently pat dry with paper towels.

To prepare sushi rice: Spread the warm rice evenly on a large rimmed baking sheet. Whisk vinegar, sugar and salt in a small bowl until the sugar dissolves; drizzle over the rice. Toss with 2 spatulas until cool enough to handle and slightly sticky, 5 to 10 minutes.

To prepare sushi: Combine mayonnaise and Sriracha in a small bowl. With wet hands, form 1 1/2 tablespoons of the sushi rice into a oval. Top with 1 tomato slice and wrap with 1 piece of nori. Repeat with the remaining tomatoes and nori. Serve topped with the Sriracha mayo, scallion greens and sesame seeds, if desired.