In a medium saucepan, combine 1 cup of water and 1 tablespoon of lemon juice, then add 4 cups of chopped tart apples like Granny Smith. Cover the pan and cook over medium heat for about 20 minutes, stirring occasionally.

Remove the saucepan from heat and mash the mixture with a potato masher or immersion blender until smooth. Strain through a fine sieve into a bowl, discarding solids.

In a separate bowl, whisk 2 whole eggs and 2 yolks until smooth. Set aside.

Return the saucepan with the apple purée to low heat. Stir in 3/4 cup of sugar, 2 tablespoons of butter, and the egg mixture while whisking continuously.

Continue cooking on low heat for 15-20 minutes, stirring until thick enough to coat the back of a spoon. Optionally, add mixed cornstarch for thicker consistency.

Once thickened, strain again through a fine sieve and allow to cool to room temperature before transferring to a jar.

Refrigerate the jar for at least 2 hours to allow the curd to set properly.