Build a bed of coals. Let the fire burn down to steady coals, not flames. Set a 12-inch camp Dutch oven over the coals to heat while you prep the squash, about 15 minutes of prep in all.
Prep the squash. Cut the squash in half lengthwise and scoop out the seeds. Brush the cut sides with 1 tablespoon of the olive oil and season with a pinch of salt.
Roast the squash in the pot. Set the halves cut-side down in the hot Dutch oven, cover with the lid, and roast over the coals for 45 minutes, until the flesh is very tender when pierced with a fork and browned at the edges. Lift the halves out and let them cool just enough to handle.
Soften the onion. Heat the remaining 1 tablespoon of olive oil in the empty Dutch oven over the coals. Add the onion and cook for 5 minutes, stirring, until soft and translucent. Add the garlic and sage leaves for the last minute, until fragrant.
Simmer it together. Scoop the roasted squash flesh into the pot and add the broth, salt, pepper, and nutmeg. Bring to a gentle boil, then lower the heat by moving the pot to fewer coals and simmer for 10 minutes, so the flavors meld.
Blend until silky. Take the pot off the direct heat. Blend with an immersion blender right in the pot until smooth. If you are using a standing blender, work in batches, fill it no more than halfway, and vent the lid so steam can escape.
Finish with cream and butter. Stir in the cream and butter and return the pot to low coals just until warmed through. Taste and adjust the salt and pepper. Serve hot, topped with pepitas or crisped sage if you like.
