Rinse the sushi rice under cold water until the water runs clear. Combine rice and water in a rice cooker or pot. Cook according to the rice cooker's instructions or bring to a boil, then reduce heat to low and cover for 20 minutes. Let it sit off heat for 10 minutes.
In a small bowl, mix the rice vinegar, sugar, and salt until dissolved. Pour this mixture over the warm rice and gently fold it in with a spatula to avoid smashing the grains. Let it cool slightly.
In a mixing bowl, combine the shredded imitation crab, diced avocado, chopped cucumber, mayonnaise, sriracha, and sesame oil. Mix until well combined. Adjust the amount of sriracha to your desired spice level.
Preheat the oven to 375°F (190°C).
Lightly grease a muffin tin with cooking spray. Press a small amount of sushi rice into the bottom of each muffin cup, creating a layer about 1/4 inch thick.
Spoon the crab mixture on top of the rice layers in each muffin cup until filled. Pack it gently but not too tight.
Place the muffin tin in the preheated oven and bake for 15-18 minutes, or until the tops are lightly golden and the filling is heated through.
Once baked, remove from the oven and let cool for a few minutes. Carefully remove the sushi cups from the muffin tin. Garnish each with small squares of nori and chopped green onions.