Mix the egg with sake and salt in a bowl to make egg mixture.

Heat a pan with a thin layer of salad oil and cook the egg mixture into a thin omelet. Roll tightly and let cool.

Mix sugar, salt, and vinegar in a bowl to make sushi vinegar.

Mix the cooked rice with sushi vinegar in a separate bowl. Reserve 130g of the seasoned rice.

Combine the reserved 130g of rice with salmon flakes to make salmon rice.

Spread 1/5 of the salmon rice onto each sheet of seaweed (20cm x 5cm) and roll tightly. Repeat for all 5 sheets.

Place the rolled salmon rice on a larger sheet of seaweed (5cm x 20cm), alternating with boiled spinach. Place the omelet in the center and roll tightly.