Wash the kabocha pumpkin thoroughly and remove the skin and seeds.
Cut the kabocha pumpkin into bite-sized pieces.
Place the pumpkin pieces in a steamer and steam for 15 minutes.
Mash the steamed pumpkin in a bowl.
Finely chop the boiled egg.
Dice the cucumber and onion finely.
Combine the mashed pumpkin, chopped egg, cucumber, onion, mayonnaise, and salt in a bowl; mix well.
Serve the completed salad on a plate.