Cut the ahi tuna into ½-inch cubes. In a large bowl whisk together the sesame oil, soy sauce, sriracha, green onions and sesame seeds.
Gently toss with the tuna and onion (if using). Cover the bowl with plastic wrap. Refrigerate the poke until ready to serve.
Place the sushi rice in a fine mesh strainer and run cool water over the rice until the water runs clear. In a saucepan or rice cooker combine the rice and 2 cups of water.
Simmer until all the water is absorbed and the rice is cooked, about 15 minutes. Fluff with a fork and gently mix in rice vinegar and mirin.