Rinse the sushi rice under cold water in a fine-mesh sieve until the water runs clear, about 2 minutes.

Combine rice and 360 ml water in a medium saucepan. Bring to a boil over high heat, then reduce heat to the lowest setting, cover, and cook for 15 minutes until all water is absorbed.

Remove from heat and let steam, covered, for 10 minutes. Spread rice onto a wide plate or bowl.

Stir rice vinegar, sugar, and salt together in a small bowl until dissolved. Fold gently through the warm rice using a rice paddle or spatula. Let cool to room temperature before building the stacks.

Pat the tuna cubes dry with paper towels. Combine sriracha, kewpie mayo, soy sauce, toasted sesame oil, and lime juice in a medium mixing bowl and whisk until smooth.

Add tuna cubes and scallions. Toss gently until each piece is coated. Cover and refrigerate for 15 to 20 minutes - no longer or the texture softens.

Toss avocado cubes with 1 tsp lime juice to prevent browning. Combine with cucumber cubes in a separate bowl and season lightly with a pinch of salt.

Place a 7 to 8 cm ring mold on a flat serving plate. Wet your hands slightly.

Spoon about 60 g of seasoned sushi rice into the mold. Press down firmly with the back of a wet spoon to form a compact base, about 1.5 cm thick.

Add a layer of cucumber-avocado mixture, about 2 heaping tablespoons, and press gently to level.

Spoon the marinated tuna on top, about 3 to 4 tablespoons, mounding it slightly in the center.

Hold the plate steady with one hand and lift the ring mold straight up in one smooth motion.

Drizzle sriracha mayo around the base of the stack. Scatter toasted sesame seeds, extra scallion slices, and nori strips over the top. Repeat for remaining servings.