Remove the skin and seeds from the kabocha pumpkin, then cut it into bite-sized pieces.
Dice the onion.
Heat olive oil in a pot and sauté the onions.
Add the kabocha pumpkin and cook together, then pour in water.
Boil until the kabocha pumpkin is cooked through.
Transfer to a blender and puree until smooth.
Season with salt and pepper.
Bring to a boil once more, then serve in a bowl.