Set up a large steamer basket or rack and steam squash whole until tender, about 1 hour. Flip the squash after 30 minutes or so. Make certain to replenish water in the steamer as needed -- do not boil the squash.
Meanwhile, heat oven to 325 degrees (F). Place walnuts on baking tray and toast in oven, stirring once or twice, until fragrant, about 15 minutes. Let cool. Reduce oven temperature to 300 degrees.
In a food processor, combine walnuts with a few Tablespoons of brown sugar and pulse a few times until nuts are coarsely ground. In a large bowl, whisk nuts with graham cracker crumbs, remaining brown sugar, lime zest, crust spices and salt. Pour melted butter over the mixture, and mix with your fingers until butter is evenly distributed. Press evenly into a 10 inch glass pie dish. Bake crust until lightly browned, about 12 minutes, then set aside. Keep oven at 300 degrees.
When squash is cool, cut it in half and scoop out seeds and pulp. Scoop flesh into a measuring cup until you have 2 1/2 cups. In a food processor, process cream cheese with sugar, pie spices, and salt until light and smooth. Scrape down bowl, add squash and process until smooth. Mix in brandy, then eggs, one at a time. Finish mixing with a rubber spatula.
Place pie plate onto a baking sheet and scrape filling into crust. Bake until just set in center, about 1 hour. Let cool before serving, topped with crème fraiche and drizzled with butterscotch sauce.
Adapted from Pichet Ong