Wash the kabocha squash thoroughly and boil or steam it with the skin on.
Let the cooked kabocha cool, then cut it into bite-sized pieces.
Finely chop the boiled eggs.
Thinly slice the cucumber and onion, then soak them in cold water to remove their sharp taste.
Place all ingredients in a bowl and mix with salt, pepper, and mayonnaise.
Transfer to a plate and serve.