Wash and halve the kabocha: Rinse the whole kabocha well, then cut it in half.
Scoop out seeds and fibers: Use a spoon to remove the seeds and the stringy bits.
Cut into pieces: Slice each half into wedges, then cut the wedges into smaller, bite-size pieces. Remove any damaged or very thick skin, but you don’t need to peel it completely as Kabocha skin is edible and becomes tender when cooked.
Arrange in the pot and add seasonings: Place the kabocha pieces skin-side up in a single layer in a pot. Add water, sugar, sake, mirin, and soy sauce (as listed in the ingredients).
Cook until tender and the liquid reduces: Bring to a gentle boil over medium-high heat. Once boiling, lower the heat to a simmer and cook about 5–7 minutes, or until the kabocha is tender when pierced with a fork and the cooking liquid has reduced to roughly half its original volume. (If pieces are large, they may need a minute or two longer.)
Serve: Transfer to a serving dish and spoon any remaining glaze over the kabocha. Serve warm.