In a large saucepan, combine chopped peeled and cored apples with water and lemon juice. Cover and cook over medium heat for about 20 minutes, stirring occasionally, until very soft.

Remove from heat and mash the apples with a potato masher. Strain through a fine mesh sieve into a bowl for a smooth apple purée.

In a separate bowl, whisk together two whole eggs and two egg yolks until smooth. Set aside.

If desired, mix cornstarch with a small amount of water to create a slurry for thickness.

Return to apple purée, adding unsalted butter and sugar. Gradually mix in the egg mixture while stirring consistently.

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

For extra smoothness, strain the mixture again through a fine mesh sieve. Allow to cool at room temperature before transferring to a jar.

Refrigerate the jar for at least 2 hours for the curd to thicken further.