For the spicy mayonnaise: Stir together the mayonnaise, Sriracha, lemon juice and salt in a bowl. Cover and refrigerate until ready to use.
For the sushi: Put the rice in a colander set over a large bowl and rinse the rice in several changes of water until the water comes out clear. Stir the rice in the colander to drain off as much water as possible, then transfer to a medium saucepan. Add 3 1/2 cups water and bring to a brisk simmer over medium-high heat. Simmer, without stirring, until much of the water has evaporated and little air vents are coming up through the rice, about 4 minutes. Cover the pot, reduce the heat to low, and cook for 15 minutes. Remove from the heat and let stand, covered, for 10 minutes. Transfer the rice to a large bowl.
Stir the vinegar, sugar and salt together in a small bowl until the sugar and salt are dissolved. Drizzle the seasoned vinegar over the rice; use a large rubber spatula to fold the rice until the vinegar is fully incorporated. Let cool until the rice is still slightly warm but cool enough to handle.
Place a bamboo rolling mat or tea towel on a work surface and cover with a piece of plastic wrap. Place a sheet of nori, shiny-side down, on the plastic, leaving about an inch of space between the front of the mat and the nori. Place about 1 cup of the vinegared rice in the center of the nori. With lightly moistened hands, press the rice evenly onto the nori, leaving a 1/2-inch border at the top and bottom. Use your finger to spread a stripe of wasabi, if using, onto the rice. Lay down a horizontal row of fillings, in any combination you like, along the bottom of third of the nori -- the fillings should only take up about an inch of width or else the roll will be overloaded. Starting from the bottom and using the mat and plastic wrap as an aid, firmly roll the sushi into a tube shape. Unmold the roll and slice crosswise into 8 equal pieces. Repeat with the remaining nori and fillings.
Arrange the sushi rolls on platters and serve with pickled ginger, soy sauce, wasabi and the spicy mayonnaise.