Rinse the sushi rice under cold water until the water runs clear. Combine the rice and water in a rice cooker and cook according to the manufacturer's instructions. Once done, let it cool slightly.

Make Tempura

In a bowl, mix the tempura batter mix with ice water until just combined. Heat the vegetable oil in a deep pan. Dip cucumber strips into the batter and fry until golden brown. Remove and drain on paper towels.

Prepare Tuna Filling

In a bowl, combine the diced tuna, chopped scallions, and a touch of soy sauce to taste.

Assemble the Roll

Place a bamboo sushi mat on a flat surface. Lay one nori sheet on the mat, shiny side down. Wet your hands and spread a thin layer of sushi rice over the nori. Leave a small border at the top. Spoon the tuna mixture and tempura cucumber along the bottom edge and roll it tightly using the mat.

Cut the Roll

Use a sharp knife to slice the roll into 6-8 pieces, wiping the knife between cuts with a damp cloth.

Serve

Arrange the sushi pieces on a plate and serve with soy sauce, wasabi, and pickled ginger.