Combine the sushi rice and water in a rice cooker and cook. When the rice is nearly finished, stir in the rice vinegar, salt and sugar. Mash the rice with a fork just a bit to ensure it's sticky.
While the rice is cooking, bring a small pot of water to boil. Have a large bowl of ice water ready. Pierce the tomatoes with a knife then drop them into the water. Boil the tomatoes until the skin begins to peel off then remove them with a slotted spoon. Immediately transfer the tomatoes to the ice water to prevent them from cooking further.
In a container with a lid, combine the soy sauce or tamari with the ginger, kelp powder, Sriracha and sesame oil. Stir until even. Peel the tomatoes and slice the flesh away from the watery seeds. Dice the tomato flesh into small pieces then transfer it to your marinade container. Place the lid on and shake it up so the tomato pieces are all coated.
Cut the cucumber and avocado into thin match sticks then prepare your sushi rolling station. On a counter or cutting board, lay out a kitchen towel with a sheet of plastic wrap on top.
Place a nori sheet on the plastic wrap. With wet fingers, spread a very thin, even layer of sushi rice on the nori. Leave an inch or so of space on either end of the sheet.
Layer spicy tomato tuna topping on the rice along with a few matchsticks of cucumber and avocado. The toppings should only cover 2 inches of the rice or you'll have trouble rolling it up.
Use your fingers to carefully peel up the edge of the sushi roll. Roll the edge up and over. As soon as the veggies are covered, roll over the plastic wrap then towel, using it to shape and compress the sushi roll. Continue until it’s all the way rolled up. Make sure to seal the edges. Cut the sushi roll with a sharp knife and enjoy!