In a medium-sized saucepan, combine 1/4 cup of water and 1 tablespoon of freshly squeezed lemon juice. Add 4 cups of chopped tart apples, ensure they're evenly coated, then cover the pot. Cook over medium heat for about 20 minutes until the apples are soft, stirring occasionally.

Remove the saucepan from heat and allow the apples to cool slightly. Using a masher, mash the apples until smooth. Strain the mashed apples through a fine sieve into a bowl, discarding any solids.

In a separate medium mixing bowl, crack and whisk together 3 large eggs and 2 egg yolks until thoroughly combined and smooth. Set aside.

In a small bowl, mix 1 tablespoon of cornstarch with 2 tablespoons of water to create a slurry. Stir until smooth and keep aside.

Return to the saucepan with the apple purée and add 1/2 cup of unsalted butter and 3/4 cup of sugar. Introduce the whisked eggs while stirring continuously over low heat. Pour in the cornstarch slurry if using.

Continue cooking over low heat for 15-20 minutes, stirring constantly, until the curd thickens and coats the back of a wooden spoon.

Strain the mixture through a fine sieve back into a clean bowl to eliminate solids. Allow to cool at room temperature for about 30 minutes before transferring to an airtight jar or container.

Refrigerate for at least 2 hours to thicken further and enhance flavors. Enjoy on toast, pancakes, or in desserts!