Using a sharp knife, cut kabocha into half and scoop out the seeds with a large spoon. Carefully peel the skin and slice the flesh into 1 to 1 ½-inch small chunks.
In a saucepan, place the kabocha and enough water to cover. Bring to a boil and cook until kabocha can be pierced with a fork without resistance but not falling apart.
Drain kabocha well and allow to cool completely.
In a large bowl, place tender kabocha and roughly mash with a fork.
Add shredded carrots, green onions, and raisins.
Add mayonnaise and gently stir to combine. Season with salt and pepper to taste.
Top with chopped walnuts and additional raisins and shredded carrots, if desired. Serve cold.
Notes
Instead of boiling, the squash can be steamed for about 12 to 15 minutes or until tender but not falling apart. Or, cooked in the microwave with about two tablespoons of water for about 4 to 6 minutes.