Preheat the oven to 400F.

Prick sweet potatoes with a fork and place on a rimmed baking sheet lined with parchment paper.

Bake until very tender, about 1 hour.

Remove from the oven and let cool slightly.

Line pie shell with parchment paper or foil and fill with pie weights or dried beans.

Bake on the lower rack of the oven until just starting to set, about 15 minutes.

Remove the weights and continue to bake until lightly golden, 10 to 12 minutes.

Cool on a wire rack 10 minutes.

Reduce the oven temperature to 350F.

Meanwhile, peel sweet potatoes and transfer flesh to the bowl of a food processor.

Discard skins.

Process until potatoes are pureed.

Transfer 1 1/2 cups puree to a large bowl (use any leftover for another use).

Add milk, sugar, eggs, cinnamon, ginger, nutmeg and salt, and whisk until combined and smooth.

Layer apple slices in concentric circles in the bottom of cooled crust.

Pour sweet potato filling into crust over apples (you may have extra filling depending on the depth of the crust).

Bake until just set in center of pie, about 50 minutes.

Let cool to room temperature on a wire rack.

Serve or chill in the refrigerator until ready to serve.