In a large bowl whisk together the flour,salt and sugar.
Add the eggs, cream, milk, and vanilla extract. Whisk until smooth.
Chill the batter in the refrigerator for at least 1 hour. (see note 4)
Assemble and bake: Get the batter out of the refrigerator and stir gently, mixing any flour that settled to the bottom during chilling.
Preheat the oven to 325° F / 170°C (150°C - if you are using a fan assisted oven)
Line a baking pan (bottom and sides) with parchment paper. Ensure it hangs over the sides of the tin for easy removal after baking. I used a 7.5 inch (19cm) round cake pan. (See note 5)
Pour the batter into the lined baking pan. Arrange the slices of apples over the batter.
Bake for 45 - 50 minutes. If the top starts to brown too quickly, lightly dome a piece of foil over it, making sure the foil doesn't touch the batter. A skewer inserted in the middle should come out clean once baked. Remember, all ovens can vary, so check a few minutes before the time is up or let it bake a little longer if needed—just keep an eye on the pie to make sure it doesn’t overcook!
Remove from the oven but leave in the pan for 15 -20 minutes to cool and set. (see note 6)
Holding on to the overhanging parchment paper, gently but quickly lift out the baked custard pie.
Slice and serve while warm or refrigerate until serving.