Slice the cucumber into 4 long strips.

Mix sugar, salt, and vinegar in a bowl to make sushi vinegar. Add the sushi vinegar to the cooked rice and mix gently to create seasoned sushi rice.

Mix egg, sake, mirin, and salt in a bowl to prepare the egg mixture. Heat a pan with salad oil and cook the egg mixture into a rolled omelette. Let it cool and slice into two pieces.

Place a sheet of roasted seaweed on a bamboo mat. Spread half of the seasoned rice evenly, leaving 3cm at the top edge.

Layer tuna, salmon, salmon roe, cucumber, and egg slices on the rice, slightly below the center. Roll tightly using the bamboo mat, pressing gently to secure the filling.