Rinse the sushi rice under cold water in a fine-mesh strainer until the water runs completely clear. Combine the rinsed rice and water in a medium saucepan, bring to a boil, then reduce heat to low, cover, and simmer for 20 minutes until the water is fully absorbed.

While the rice is still warm, transfer it to a large wooden or glass bowl. Gently fold in the rice vinegar, sugar, and salt with a spatula until the rice is evenly coated and glossy, then let it cool to room temperature.

In a small mixing bowl, combine the finely chopped sushi-grade tuna, mayonnaise, sriracha sauce, and sesame oil. Mix gently until the tuna is fully coated in the creamy, spicy sauce.

Place a bamboo sushi mat on a flat work surface and lay one sheet of nori rough-side up. Wet your hands with a little water to prevent sticking, then spread an even layer of the cooled sushi rice over the nori, leaving a 1-inch bare border at the top edge.

Arrange a horizontal line of the spicy tuna mixture and a few strips of julienned cucumber across the lower third of the rice-covered nori.

Carefully lift the bottom edge of the bamboo mat and roll it tightly over the filling, applying gentle, even pressure to shape it into a cylinder. Wet the exposed top border of the nori with a dab of water to seal the roll completely.

Using a very sharp, wet knife, slice the sushi roll into 6 to 8 even pieces, wiping the blade with a damp cloth between cuts to ensure clean slices. Garnish with toasted sesame seeds and serve immediately with soy sauce, pickled ginger, and a dab of wasabi.