Preheat oven to 425 °F (220 °C).
Cut the squash into quarters (see blog post above to learn how to cut kabocha safely). Cut the onion into quarters, leaving the skin on. Chop the ginger and peel the garlic cloves.
Line a baking tray with parchment paper. Spread the kabocha squash, onion, ginger and garlic on the tray.
Drizzle with 1 tablespoon maple syrup, the olive oil, and the maple syrup. Rub it on to the squash evenly with your hands. Sprinkle 1/2 teaspoon of salt over it.
Turn the squash and onions to be skin-side-up. Make sure the garlic cloves are under the hoods of the squash to help them keep moist.
Roast for 1 hour at 425 °F (220 °C). Flip the squash and onions at the 30 minute mark, still skin-side-up, and the other cut side down.
Once the vegetables are done, let them cool for 5 to 10 minutes on the kitchen counter, until you can handle the squash with your hands.
Using a knife, peel the skin off the squash and discard it. Remove the dried layer from the onions and discard it.
Add the roasted veggies, the remaining 1 tablespoon maple syrup, and the remaining 1 teaspoon salt, along with the cumin, vegetable broth, and butter, to the blender, Blend on high until smooth, about 5 minutes. Taste the soup. Adjust the texture by adding more broth as desired, and adjust the seasoning by adding more salt if needed. Then blend again to mix well.
Garnish with olive oil (or heavy cream), chopped sage or thyme, and freshly ground black pepper, if needed. Serve hot with toasted bread. Enjoy!