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

Use a potato masher to mash the cooked apples into a chunky purée, then strain through a fine sieve back into the saucepan.

Whisk together 2 large eggs and 2 egg yolks until frothy in a separate bowl.

For a thicker curd, mix 1 tablespoon of cornstarch with 2 tablespoons of water until smooth.

Add 1/2 cup unsalted butter and 3/4 cup sugar to the purée, mixing continuously over low heat. Stir in the prepared egg mixture and cornstarch slurry.

Cook on low heat for 15 to 20 minutes, stirring gently until thickened.

Strain the curd again through a fine sieve, cool for about 15 minutes, then transfer to an airtight container.

Refrigerate for at least 2 hours before using.