Bring a large pot of salted water to a boil. Using a small sharp knife, carefully cut a 2-inch circle around the stems of the squash and pull them out. Discard any seeds that cling to the tops; reserve the tops. Scoop out and discard the remaining seeds in-side the squash. Cook the squash and tops in the boiling water until the flesh is just tender when pierced, 10 to 15 minutes. Transfer to a bowl of ice water to cool, then drain upside down on paper towels
Carefully scoop the flesh from each squash into a bowl, leaving a 1-inch-thick shell; do not pierce the squash, or they will leak. You should have about 4 cups of squash flesh; if necessary, scoop the flesh from the tops.
In a large saucepan, cook the pancetta over moderately high heat, stirring frequently, until lightly browned, about 5 minutes. Transfer the pancetta to a plate.
Add the onions to the saucepan and cook over low heat, stirring occasionally, until transparent, about 6 minutes. Add the Ham Stock and bring to a boil over high heat. Stir in the squash flesh and bring back to a boil. Transfer the soup to a blender or food processor and puree until smooth. Strain into the saucepan and season with salt and white pepper. If necessary, stir in a little hot water to thin the soup; keep warm.
Preheat the oven to 350°. Arrange the squash shells on a baking sheet and spread the pancetta alongside. Warm in the oven for about 5 minutes, or until the bowls are heated through.
Set the squash shells on individual plates and ladle in the hot soup. Garnish with the pancetta and chives and serve with the squash lids on top of, or alongside, the shells.
The recipe can be prepared to Step 4 up to 2 days ahead; cover and refrigerate the soup, squash shells, and pancetta separately. Reheat the soup before serving.