Preheat oven to 350°F.
Slice squash in half and remove seeds. Place cut side down on a parchment-lined baking sheet. Bake for 45-60 minutes, or until tender. Let cool.
Scrape flesh from squash and transfer to a food processor along with eggs, cream, Swiss cheese, salt, and pepper. Process until smooth.
Butter a 6-cup gratin pan or baking dish and fill with mixture. Top with grated Parmesan.
Bake for 45-60 minutes, or until top is nicely browned and gratin is set.