Cut the ends off the kabocha squash to stabilize it. Cut it in half lengthwise with a sharp knife. Use a spoon to remove the seeds and the fibers.
Place the two halves of the kabocha cut side up on a baking sheet lined with parchment paper. Lightly brush with olive oil and season with salt and pepper. Turn the pieces cut side down.
With a sharp knife, cut the top off each head of garlic (about 5 mm) to expose the cloves.
Place the heads of garlic cut side up in the dish.
Bake for a total cooking time of 60 minutes.
After 30 minutes, cut an onion in half, brush it with oil and place it on the tray cut side down. Continue cooking with the onion for the remaining 30 minutes.
When removing from the oven, let the kabocha cool slightly before scooping out the flesh with a spoon.
Peel the onion to remove the burnt outer layer, then add its softened flesh to the blender along with the kabocha flesh, garlic, vegetable broth, and a little salt.
Mix until smooth and creamy.