Adjust the rack of the oven to the lower-middle level and heat the oven to 325*F.
Melt butter in a 12-inch skillet over medium heat.
Add the shredded apples and cook, stirring frequently, until the liquid in the pan is gone and the pan is dry, and the apples have softened, about 12-15 minutes.
Transfer the apples to a bowl and allow them to cool a bit, about 20 minutes.
Whisk together the sugar, cinnamon, mace and salt in a large bowl.
Add the eggs, cream, sherry, lemon zest, and vanilla and whisk together until smooth.
Add in the cooled apples and stir to combine.
Pour the apple custard mixture into a pre-baked pie shell.
Bake until the center is set, about 40 minutes.
Cool completely on a wire rack before serving, about 4 hours.
Can be made a day ahead and chilled.
Serve with whipped cream.