California Rolls

Name: California Rolls
Ingredients: 4 ounces crabmeat or surimi (imitation crab); 1 to 2 tablespoons mayonnaise (optional); 1/4 cup rice vinegar; 2 1/4 cups Sushi Rice, recipe follows; 5 sheets nori, halved lengthwise; Wasabi paste, as needed, plus more for serving; 1 Japanese or hothouse cucumber, cut into strips; 1 avocado, cut into strips; Sriracha (optional); Toasted sesame seeds, for sprinkling; Soy sauce, for dipping; 3 cups (750 grams) sushi rice; 3 cups (750 grams) water; 6 ounces (120 ml) unseasoned rice vinegar; 5 tablespoons (75 grams) sugar; 1/4 cup (50 grams) sea salt or kosher salt; One 4-inch square kombu