Step 1Heat oven to 400 degrees. Split the squash in half lengthwise and scoop out the seeds. Place cut-side up on a cookie sheet and roast until it can be easily pierced with a paring knife, about 1 hour. Remove from heat and cool slightly.
Step 2When cool enough to handle, use a spoon to scoop out the squash meat, leaving behind the papery shell. Put the meat in a food processor with the pear, shallot and butter and puree until smooth. Pulse in the salt, dried mustard, white pepper and crushed amaretti. Pulse in the balsamic vinegar. Taste and add more salt or vinegar if necessary. (The recipe can be prepared to this point up to 1 day in advance and refrigerated tightly covered.)
Step 3When almost ready to serve, transfer the puree into a saucepan and warm over medium-low heat until it bubbles and thickens. Serve immediately.