Remove the seeds from the kabocha squash with a spoon.

Chop the kabocha squash into bite-sized pieces.

Shave off the corners of each piece to prevent them from falling apart during simmering.

Place the kabocha squash pieces into a pot in a single layer.

Add water until the squash is just covered.

Bring the water to a boil, then add the light brown sugar and sake.

Cover the pot and simmer for 3 minutes.

Observe the color change of the kabocha squash.

Add the mentsuyu and simmer for an additional 5 minutes over medium-low heat, adjusting the heat to prevent rapid evaporation.

If needed, add more water to the pot along the sides to prevent the squash from drying out.

Just before turning off the heat, add the mirin and soy sauce.

Remove the pot from the heat, replace the lid, and let the squash steam for a few minutes.

Serve the simmered kabocha squash.