Preheat oven to 350°F. Place unbaked pie shell in a 9-inch pie plate and set on a baking sheet.
In a large bowl, whisk eggs and sugar until smooth. Add melted butter, applesauce, flour, vanilla, cinnamon, nutmeg, and lemon juice (if using). Whisk until well combined.
Pour filling into the unbaked pie shell.
Add Crumble Topping (Optional)
In a small bowl, combine flour, brown sugar, and cinnamon. Cut in cold butter with a fork until the mixture resembles coarse crumbs.
Sprinkle crumble topping evenly over the pie filling.
Bake and Cool
Bake for 40-45 minutes until the filling is set but the center still jiggles slightly. Cover crust edges with foil if they brown too fast.
Cool to room temperature, then refrigerate for at least 2 hours before slicing. Serve cold for the best texture.
Notes
Serve cold: This pie is best served chilled. The custard sets up in the fridge and slices cleanly.
Unsweetened applesauce: If using sweetened applesauce, reduce sugar to ½ cup.
Don't overbake: The center should jiggle slightly when done. It firms up as it cools.
Storage: Cover and refrigerate up to 3-4 days. Freeze individual slices wrapped tightly for up to 3 months.
Crust: Store-bought or homemade pie crust both work great.