Cut off the ends of the cucumber. Use a peeler to slice it thinly, then cut into 1cm wide strips.

Place the tuna in a tray and pour the concentrated soy sauce over it. Cover with plastic wrap and marinate in the refrigerator for 30 minutes.

In a bowl, mix the cooked rice with sushi vinegar using a cutting motion.

In another bowl, crack the eggs and add a pinch of salt. Mix in the cornstarch slurry (cornstarch and water).

Heat a thin layer of salad oil in a frying pan. Pour one-third of the egg mixture and cook over low heat, shaping it as it cooks.

Flip and cook both sides. Repeat to make a total of three egg sheets.

Lay the egg sheets slightly overlapping vertically. Place the marinated tuna on top with some spacing and roll tightly from the edge.

On a plastic wrap, arrange the cucumber strips in a grid pattern and braid them.

Place plastic wrap on a sushi mat and spread the sushi rice evenly. Place the rolled egg and tuna slightly closer to the edge and roll tightly.

Place the braided cucumber on the sushi mat, then place the sushi roll on top. Roll tightly and wrap in plastic wrap. Refrigerate for 10 minutes.