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 1½ teaspoons of rice vinegar and mirin.
Cut the ahi tuna into ½-inch cubes.
In a large bowl whisk together the sesame oil, soy sauce, the remaining 1½ teaspoons of rice vinegar, 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.
Divide the warm rice into serving bowls. Top with scoops of the chilled poke and avocado and serve.
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Notes
Try adding favorites like edamame, mango, crispy onions, fried shallots, macadamia nuts, furikake, chili oil, pickled ginger, and seaweed salad for unique flavor combinations!