Fill about half of a mixing bowl with water and your lemon juice, and let it mix it a bit.
1 Lemon
Peel the apples, and be sure to take out the pits!
2 Apples
Cut your apples into small cubes (about 0.5 cm / 0.2 inch). Drop the apple cubs into the bowl with the lemon water.
In a small pan, combine 50 grams / ¼ cup of sugar and 5 tbsp. of the lemon water.
¾ cup Granulated Sugar
Heat up the sugar and lemon water on medium heat until the sugar fully dissolves.
Strain the apples from the rest of the lemon water mixture into a pan. You'll also be adding in the cinnamon and the roughly chopped walnuts in the pan.
1 tablespoon Cinnamon, ½ cup Walnuts
Simmer them together on medium heat until the apples start to caramelize and begin to soften (about 5-10 minutes).
Let them cool down on the side.
In a separate medium bowl, whisk the egg yolks, whole eggs and sugar together, leaving enough space for the milk and heavy cream.
3 Egg Yolks, 3 Eggs, ¾ cup Granulated Sugar
Add in the corn starch to the mix and whisk well — you don't want to see any starch clumps!
¼ cup Corn Starch
Once that's been whisked well, you'll need to combine the milk, heavy cream and lemon zest (optional) in a larger pot. Be sure to leave some extra space in this case, as well!
1 ½ cup Whole Milk, ¾ cup Heavy Cream
Bring everything to a simmer, and then slowly pour it into your bowl with the egg and corn starch mix.
It's important that you're vigorously whisking while pouring!
Next, pour the mix back in the pot and cook on low to medium heat, all while constantly whisking until it starts to thicken. When the whisk starts to form ribbons, add in the vanilla extract and caramelized apples and walnuts. Take it off the flame.
1 tablespoon Vanilla Extract
Make sure you don't over fill the cake pan, though, by leaving roughly 1 cm / 0.4 inch empty.
You'll need to bake your delicious custard apple tart at 180 °C / 356 °F for at least 40 minutes. For an indicator that your pastry is ready to come out of the oven, check if the surface of the custard starts to turn a golden brown. When you do take it out of the oven, the cake should be jiggly when shaken.
Let the cake to cool completely at room temperature before taking out the cake pan ring.
In order to really impress your guests, mix one tbs. of cinnamon with one tbsp. of icing sugar before sieving it over the cake!