Combine 1 cup of water and 2 tablespoons of lemon juice in a large saucepan. Add 4 cups of chopped tart apples and cook over medium heat for about 20 minutes until soft.

Mash the apples using a potato masher until smooth, then strain through a fine mesh sieve to achieve a silky apple purée. Return to the saucepan.

Whisk together 2 whole eggs and 2 yolks in a separate bowl until smooth and frothy, then set aside.

If desired, mix 1 tablespoon of cornstarch with 2 tablespoons of water until smooth. Keep for later.

In the saucepan with apple purée, add 1/2 cup of sugar and 4 tablespoons of butter. Gradually stir in the whisked eggs until mixed.

Cook over low heat, stirring constantly for 15-20 minutes, until the mixture thickens to coat the back of a spoon.

Strain the mixture again through a fine mesh sieve for a smoother texture. Let cool to room temperature before storing.

Refrigerate the strained apple curd for at least 2 hours before serving to allow flavors to meld.