Rinse sushi rice until water runs clear. Cook with water in rice cooker or saucepan. Let steam off heat for 10 mins.

Heat vinegar, sugar, and salt until dissolved. Gently fold mixture into hot rice in a wide bowl while fanning. Cover with damp cloth.

Mix shredded crab, 3 tbsp mayo, sriracha, sesame oil, and most of the green onion until creamy. Set aside.

In another bowl, whisk 1/2 cup mayo, sour cream, 2-3 tbsp sriracha, tobiko (if using), and soy sauce for the topping. Set aside.

Place a nori sheet on bamboo mat. Spread ~3/4 cup rice evenly, leaving a 1-inch border at top. Add crab filling, cucumber, and avocado along bottom edge.

Roll tightly using the mat, sealing at the end. Repeat to make 5 rolls.

Slice each roll into 8 pieces with a wet, sharp knife. Arrange pieces cut-side up on a parchment-lined baking sheet.

Preheat broiler on high. Spoon or pipe the creamy volcano topping generously over each piece.

Broil 6 inches from heat for 3-5 minutes until topping is bubbling and has browned spots. Watch closely.

Garnish immediately with sesame seeds and remaining green onion. Serve hot with soy sauce, pickled ginger, and wasabi.