Step 1: Rinse the sushi rice thoroughly under cold running water until the water runs clear. Drain well.
Step 2: Combine rinsed rice and water in a medium saucepan. Bring to a boil over high heat, then immediately reduce heat to low, cover, and simmer for 15 minutes, or until all water is absorbed. Do not lift the lid during cooking.
Step 3: Remove from heat and let stand, covered, for 10 minutes.
Step 4: While the rice rests, prepare the sushi vinegar seasoning: In a small bowl, whisk together rice vinegar, sugar, and salt until sugar and salt are dissolved.
Step 5: Transfer the cooked rice to a large, non-metallic bowl or a wooden hangiri. Pour the sushi vinegar seasoning over the rice. Using a cutting motion with a rice paddle or wooden spoon, gently fold the seasoning into the rice. Do not mash. Fan the rice (with a fan or piece of cardboard) as you mix to cool it quickly and give it a glossy finish. Let the rice cool completely to room temperature before using.
Step 6: Prepare the spicy crab filling: In a medium bowl, combine the shredded imitation crab meat, Japanese mayonnaise, sriracha (start with 2 tablespoons and add more for extra heat), sesame oil, and optional garlic powder. Mix well until everything is evenly coated and creamy.
Step 7: Assemble the rolls: Place a bamboo sushi mat on a clean surface. Lay one sheet of nori, shiny side down, on the mat. Moisten your hands with water (this prevents rice from sticking). Take about 3/4 to 1 cup of cooled sushi rice and gently spread it evenly over the nori, leaving a 1/2-inch strip of nori exposed at the top edge.
Step 8: Arrange a line of spicy crab mixture and cucumber matchsticks (and avocado, if using) horizontally across the center of the rice-covered nori. Don't overfill.
Step 9: Using the bamboo mat, carefully lift the edge closest to you and begin to roll tightly, pressing down gently to compact the ingredients. Continue rolling, pulling the mat away from the roll as you go, until the roll is sealed by the exposed nori strip at the top. You can wet the exposed edge slightly to help it seal.
Step 10: Repeat with the remaining nori sheets and filling.
Step 11: To slice: Use a very sharp, damp knife to cut each roll into 6-8 pieces. Wipe the knife clean and re-dampen between each cut for the cleanest slices.
Step 12: Arrange the spicy crab roll pieces on a serving platter. Garnish with toasted sesame seeds and thinly sliced green onions, if desired. Serve immediately with soy sauce, wasabi, and pickled ginger. Enjoy your homemade spicy crab roll!