Position a rack in the lower third of the oven and preheat to 325ºF.
In a medium bowl, whisk together the sugar, pumpkin spice, and salt.
Whisk in the eggs and egg yolks, then the maple syrup, squash puree, crème fraîche or sour cream, milk or cream, and vanilla.
Strain the filling through a medium-mesh sieve to get it super smooth and remove any eggy bits.
Pour the filling into the parbaked crust and bake at 325º until the tart wobbles slightly like Jell-o when you give it a wiggle, 30-40 minutes.
Cool completely, 1 hour, then chill until firm, at least 2 hours. The tart is best the day of baking when the crust is crisp, but it keeps well, refrigerated airtight, for up to several days.
To serve, remove the sides from the tart pan and transfer the tart to a serving platter. Top the tart with the whipped crème fraîche, sprinkle with toasted nuts, and drizzle with a bit of maple syrup. Cut into wedges and serve.
Notes
Substitution suggestions:
In place of sugar, use maple sugar or coconut sugar for refined sugar-free
In place of golden pumpkin pie spice, use ½ teaspoon ground cinnamon, ½ teaspoon ground ginger, ¼ teaspoon freshly grated nutmeg, 1/8 teaspoon ground allspice, 1/8 teaspoon ground turmeric (optional, mostly for color)
In place of creme fraiche and milk, use 1 cup coconut cream (the creamy part from a can of coconut milk)
In place of maple syrup, use more granulated sugar or brown sugar
Make-Ahead Options:
The unbaked crust can be shaped, tightly wrapped, and frozen for up to several months.
The baked crust can be wrapped and refrigerated for up to 3 days.
Squash puree can be made and refrigerated, covered, up to 1 week ahead, or frozen for up to several months.
The tart is best the day of baking when the crust is crisp, but leftovers keep well, refrigerated airtight, for several days, or frozen for up to 2 months.