Prepare the batter:
In a large bowl, whisk together the flour, sugar, and salt. Add the eggs, cream, milk, and vanilla extract. Whisk until smooth.
Chill the batter:
Refrigerate the batter for at least 1 hour to allow the ingredients to meld and the flour to hydrate.
Preheat and prepare the pan:
Preheat your oven to 325°F (170°C) or 150°C if using a fan-assisted oven. Line a 7.5-inch (19 cm) round cake pan with parchment paper, ensuring it hangs over the sides for easy remorval after baking.
Assemble the pie:
Remove the chilled batter from the refrigerator and stir gently to mix any flour that may have settled at the bottom. Pour the batter into the prepared pan. Arrange the thinly sliced apples evenly over the top of the batter.
Bake the pie:
Bake for 45-50 minutes, checking for doneness by inserting a skewer into the middle. It should come out clean when the pie is done. If the top starts to brown too quickly, lightly cover with foil, ensuring the foil does not touch the batter.
Cool and serve:
Once baked, remove the pie from the oven and let it cool in the pan for 15-20 minutes to set. Use the overhanging parchment paper to lift the custard pie out of the pan. Slice and serve warm, or refrigerate until ready to enjoy.