01 - Rinse sushi rice under cold water until water runs clear. In a saucepan, combine rice with 1 1/4 cups water. Bring to a boil, cover, reduce heat to low, and cook for 15 minutes. Remove from heat and let rest, covered, for 10 minutes.
02 - Mix rice vinegar, sugar, and salt in a small bowl. Gently fold mixture into the cooked rice. Spread rice onto a parchment-lined baking sheet and shape into a rectangle about 1 inch thick. Allow to cool completely, then refrigerate for at least 30 minutes or up to overnight until very firm.
03 - Combine diced tuna, mayonnaise, Sriracha, soy sauce, sesame oil, scallions, and lemon juice in a bowl. Mix gently. Cover and refrigerate until needed.
04 - Once rice is firm, cut into 16 rectangles or squares. Heat neutral oil in a non-stick skillet over medium-high heat. Fry rice pieces in batches, 2 to 3 minutes per side, until golden and crispy. Transfer fried rice to paper towels to drain excess oil.
05 - Arrange crispy rice cakes on a serving platter. Top each with a spoonful of spicy tuna mixture. Garnish with sesame seeds, sliced scallions, avocado, and microgreens or nori strips if desired. Serve immediately.