Prepare the Sushi Rice:

1. Cook the rice: Rinse sushi rice under cold water in a fine-mesh strainer until the water runs clear (this removes excess starch). Cook rice according to package directions or in a rice cooker with 2½ cups water. Once cooked, let it steam for 10 minutes with the lid on.
2. Season the rice: While rice cooks, mix rice vinegar, sugar, and salt in a small bowl until dissolved. When rice is done, transfer to a large bowl and gently fold in the vinegar mixture and sesame oil using a cutting motion (don’t stir or smash the rice). Let the rice cool to room temperature, about 30 minutes.
3. Press the rice: Line an 8×8-inch baking dish or small loaf pan with plastic wrap, leaving overhang on the sides. Transfer the cooled rice into the pan and press down firmly and evenly using wet hands or the bottom of a measuring cup. You want a compact, even layer about ¾ to 1 inch thick. Cover with plastic wrap and refrigerate for at least 20 minutes (or up to overnight) to firm up.

Prepare the Spicy Tuna:

4. Dice the tuna: Pat the tuna dry with paper towels. Using a sharp knife, cut the tuna into small, uniform dice (about ¼-inch cubes). Place in a medium bowl and refrigerate until ready to use.
5. Make spicy mayo: In a small bowl, whisk together mayonnaise, sriracha, sesame oil, and soy sauce until smooth. Taste and adjust spice level.
6. Mix spicy tuna: Just before assembling, add the spicy mayo mixture to the diced tuna along with chopped green onions (white parts). Gently fold together until tuna is evenly coated. Keep refrigerated until ready to serve.

Cook the Crispy Rice:

7. Cut the rice: Remove the rice block from the pan using the plastic wrap overhang. Unwrap and place on a cutting board. Using a sharp knife dipped in water, cut the rice into 12-16 rectangular pieces (about 2 x 1.5 inches each). Wipe the knife clean between cuts for neat edges.
8. Heat the oil: Heat 2-3 tablespoons of neutral oil in a large non-stick skillet over medium-high heat until shimmering but not smoking.
9. Fry the rice: Working in batches, carefully place rice rectangles in the hot oil, flat side down. Don’t overcrowd the pan! Fry for 3-4 minutes without moving them, until the bottom is deeply golden and crispy. Carefully flip and fry the other side for another 2-3 minutes. Transfer to a paper towel-lined plate. Add more oil between batches as needed.

Assemble & Serve:

10. Build the bites: Arrange crispy rice pieces on a serving platter. Top each piece with a generous spoonful of spicy tuna mixture. Add a thin slice of avocado on top of the tuna.
11. Garnish: Drizzle with eel sauce and extra spicy mayo if desired. Sprinkle with sesame seeds, sliced green onions, and furikake. Add microgreens for an elegant touch.
12. Serve immediately: These are best enjoyed right away while the rice is still warm and crispy! Provide small plates and chopsticks or forks.