Make the pie dough ahead of time so it has time to rest for at least 2 hours, but ideally overnight, then shape it into the pie dish at least 30 minutes before baking. Full instructions in my Flaky Pie Crust post. Alternatively, you can use a store-bought pie crust.
Cover the inside of the pie with parchment paper and fill with baking beans and/or rice. Make sure you fill it all the way up the sides of the pie crust to stop the edges from shrinking.
Keep the pie crust in the fridge, or if there is space, pop it in the freezer for 10 minutes while you preheat the oven to 425°F / 220°C (200°C fan assisted).
Put the pie crust into the oven on the middle rack and bake for 15-18 minutes or until the edges are starting to turn golden brown.
Take the pie out of the oven and turn the temperature down to 375°F / 190°C (170°C fan assisted). Carefully lift out the parchment paper and baking beans.
Brush a layer of egg wash on the bottom of the pie, it should be a thin layer, but thick enough to seal any docking holes. Return the pie crust to the oven for a further 5-7 minutes, or until the inside looks dry and light golden brown.
Remove from the oven and let the pie crust cool while you make the custard filling.
Make the Custard Filling
Set the oven temperature to 325°F / 160°C (140° fan-assisted).
In a large bowl, whisk together the eggs, egg yolk, sugar, and salt. Whisk gently just until combined - don't try to incorporate any air into the mixture.
To a saucepan, add the heavy cream, milk, and vanilla, whisk together, and warm over medium-low heat until it is scalding. (This means there is steam coming from it and you can see a few tiny bubbles starting to form on the surface. Don't let it boil.
Slowly drizzle a tiny amount of the scalded milk/cream into the egg mixture and whisk it in. Keep adding it, a little at a time, until it is all incorporated.It is vital to do this gradually to slowly bring up the temperature of the eggs so that they don't scramble.
Pour the custard mixture through a fine mesh sieve into a large bowl to remove any lumps of coagulated egg. Tip: If you have a bowl with a spout on it, it makes the next step a lot easier!
Pour the custard filling into your par-baked pie crust and sprinkle with nutmeg. If you would like, brush the edges of the pie crust with a little of the eggwash.
Carefully transfer the custard pie to the middle rack of your oven and bake for 40- 50 minutes. If the pastry starts to brown too much but the pie isn't ready, you can add a pie shield to stop it from browning further.
Bake time will vary massively depending on a variety of factors, so it's best to know the signs of when it is done. Tap or move the pie dish very slightly. When custard pie is properly baked, the outer 1-2 inches of pie will be set and firm, but the middle will still jiggle. See the post above for more details.
Remove the custard pie from the oven and let it cool to room temperature before transferring it to the refrigerator to chill for at least 4 hours - preferably overnight.
Notes
Pie Crust
I recommend making homemade pie dough from scratch, but if you don't want to make it from scratch, you can buy a store-bought pie crust. It should be 9 inches and deep dish.