Step 1: Rinse 1.5 cups of short-grain sushi rice under cold water until the water runs clear. This usually takes about 5-7 rinses. Drain well.
Step 2: Cook the rice: Combine the rinsed rice with 2 cups of cold water in a medium saucepan. Bring to a boil over high heat, then reduce heat to low, cover tightly, and simmer for 15 minutes. Remove from heat and let stand, covered, for another 10 minutes. Do not lift the lid!
Step 3: Prepare the sushi seasoning: In a small bowl, whisk together 3 tablespoons of rice vinegar, 1 tablespoon of granulated sugar, and 1 teaspoon of salt until dissolved.
Step 4: Season the rice: Transfer the cooked rice to a large, non-metallic bowl. Pour the sushi seasoning evenly over the hot rice. Using a rice paddle or wooden spoon, gently fold the seasoning into the rice using a cutting motion, being careful not to mash the grains. Fan the rice (with a small fan or piece of cardboard) as you fold to cool it quickly and give it a glossy sheen. Continue until the rice is lukewarm.
Step 5: Press the rice: Line a small baking dish (around 8×8 inches or similar) or a baking sheet with plastic wrap, leaving an overhang. Lightly wet your hands and firmly press the lukewarm sushi rice into an even layer, about 1/2 to 3/4 inch thick. Cover with the plastic wrap overhang and refrigerate for at least 1 hour, or up to 4 hours, to allow it to firm up.
Step 6: Cut the rice: Once chilled, remove the rice block from the dish using the plastic wrap. Peel off the plastic. Using a sharp knife dipped in water (to prevent sticking), cut the rice into 12-16 rectangular or square pieces (approx. 1×2 inches or 1.5×1.5 inches). Wipe the knife between cuts.
Step 7: Fry the crispy rice: Heat 1/2 inch of neutral oil (like vegetable, canola, or grapeseed oil) in a large non-stick skillet over medium-high heat until it shimmers (around 350°F / 175°C). Carefully place the rice pieces into the hot oil, leaving space between them. Fry for 3-5 minutes per side, or until deeply golden brown and crispy. Work in batches to avoid overcrowding the pan. Transfer the crispy rice pieces to a wire rack lined with paper towels to drain any excess oil.
Step 8: Prepare the tuna: Finely dice 8 ounces of sushi-grade tuna. Aim for small, uniform pieces. Transfer to a medium bowl.
Step 9: Mix the spicy mayo: In a separate small bowl, combine 1/4 cup Japanese mayonnaise (Kewpie), 2 tablespoons Sriracha (adjust to taste), 1 teaspoon soy sauce, and 1/2 teaspoon sesame oil. Whisk until smooth.
Step 10: Combine: Add the spicy mayo mixture to the diced tuna. Gently fold until the tuna is evenly coated. Taste and adjust seasoning if needed.
Step 11: Assemble: Top each crispy rice piece with a generous spoonful of the spicy tuna mixture. You can use a small spoon or a piping bag for a neater presentation.
Step 12: Garnish and serve: Garnish with thinly sliced jalapeño, a sprinkle of black or white sesame seeds, and a few microgreens or finely chopped chives, if desired. Serve immediately.