Step 1: Rinse sushi rice under cold water until water runs clear. Cook rice with water in a rice cooker or pot according to package instructions.
Step 2: In a small saucepan, heat rice vinegar, sugar, and salt until dissolved. Gently fold into cooked rice and let cool to room temperature.
Step 3: In a bowl, mix imitation crab, mayonnaise, sriracha, and soy sauce to create the volcano topping.
Step 4: Place a nori sheet on a bamboo sushi mat. Spread about 1/2 cup seasoned rice evenly over nori, leaving 1-inch border at top.
Step 5: Arrange avocado slices and cucumber strips horizontally across the center of the rice.
Step 6: Roll sushi tightly using the mat, moistening the nori border with water to seal.
Step 7: Repeat with remaining nori sheets and fillings to make 4 rolls.
Step 8: Slice each roll into 8 pieces and arrange on a baking sheet.
Step 9: Top each piece with a spoonful of the crab mixture and chopped shrimp.
Step 10: Broil in oven for 2-3 minutes until topping is bubbly and lightly browned.
Step 11: Garnish with sesame seeds and tobiko before serving.