Start by rinsing 1 cup of sushi rice under cold water until the water runs clear. In a rice cooker, combine the rinsed rice with 1 1/4 cups of water and a pinch of salt. Cook according to the rice cooker's instructions, usually about 20 minutes.

Once the rice is cooked, allow it to cool for about 10 minutes. In a small bowl, whisk together 3 tablespoons of rice vinegar, 1 tablespoon of sugar, and 1 tablespoon of sesame oil until dissolved. Gently fold this mixture into the cooled rice using a spatula.

Spread the seasoned rice mixture evenly into a lined baking tray, pressing it down firmly to form a compact block. Cover and refrigerate for at least 1 hour to firm up the rice.

After chilling, carefully lift the rice block from the tray and remove the lining. Using a sharp knife, slice the rice into rectangular pieces, aiming for around 2-inch by 3-inch squares.

In a non-stick skillet, heat 1/4 cup of vegetable oil over medium heat until shimmering. Fry rice rectangles in batches for about 2-3 minutes on each side until golden brown and crispy. Drain on paper towels.

Prepare the spicy tuna mixture by combining 8 ounces of diced sushi-grade ahi tuna, 2 tablespoons of Japanese Kewpie mayonnaise, 1 tablespoon of sriracha, 1 teaspoon of soy sauce, and 1 teaspoon of sesame oil. Add lime juice to taste and mix gently.

Place a slice of avocado on top of each golden rice rectangle, then spoon the spicy tuna mixture over the avocado. Finish with toasted sesame seeds and garnish with chives or jalapeño slices.