Drain the canned mackerel. Slice the cucumber thinly, sprinkle with a pinch of salt, and knead gently. Let it sit for 5 minutes until softened, then squeeze out excess water.

In a bowl, mix vinegar, sugar, and salt to make sushi vinegar. In another bowl, combine warm rice, sushi vinegar, and sesame seeds. Mix gently while fanning the rice to give it a glossy finish. Cover with plastic wrap and let it cool to room temperature.

Beat the eggs with potato starch to make an egg mixture. Heat salad oil in a pan over medium heat, spread it thinly with a paper towel, and pour a thin layer of the egg mixture. Once set, remove and repeat until all the mixture is used. Roll the cooked egg sheets and slice thinly to make shredded egg (kinshi tamago).

Wipe the pan clean and add the canned mackerel along with sake, mirin, sugar, and soy sauce. Cook over medium heat while breaking the mackerel into small pieces. Simmer until the liquid evaporates to make mackerel soboro.