Preheat oven to 400 degrees. Pierce potatoes several times with the tines of a fork. Roast until tender when pierced with the tip of a knife, about 1 hour. Set aside to cool slightly. When cool enough to handle, peel, discarding skins, and place flesh in a medium bowl. Mash well; set aside to cool.
In a medium bowl, whisk eggs, sugar, lemon zest, cinnamon, allspice, nutmeg, and salt, until well combined. Whisk in cream.
Add egg mixture to cooled sweet potatoes, and whisk until thoroughly combined. Whisk in melted butter. Pour into baked crust. Bake until filling is set and the tip of a knife comes out clean when inserted into the center, 45 to 50 minutes. Cool completely before slicing. Serve topped with whipped cream.