Core, peel, coarsely chop the apples, put them in a small to medium sized pot with a small amount of water to cover the bottom. Heat over medium low heat until the apples start to break down, about 15 minutes.
2 apples
Blitz the softened apples in a food processor or run them through a food mill until smooth.
Add the egg and egg yolk to a small bowl and whisk until blended. Set aside.
1 egg, 1 egg yolk
Add the sugar, lemon juice, butter cubes, salt, and ¾ cup of the puréed apple to a medium sized pan, ideally one with sloping sides. Heat on medium-low heat until the butter has melted and then remove it from the heat.
½ cup (100 grams or 3.5 ounces) granulated sugar, ¼ cup lemon juice, 4 tablespoons (57 grams or 2 ounces) unsalted butter, cut into cubes, 1 pinch salt
Temper the eggs by spooning a little bit of the warm apple mixture into the eggs, whisking as the mixture is added. Keep spooning a little bit at a time, whisking constantly, until the eggs have warmed.
Now add the egg to the pot with the apple mixture, again whisking constantly.
Once all of the egg has been added, return the pot to medium-low heat, stirring constantly until the mixture has thickened. The classic test of doneness is when the custard coats the back of a spoon, and you can draw a line through the curd with your finger. This should take about 10 to 15 minutes but may take longer if you have the heat low. Do not let the apple curd come to a boil or start to steam. If it does, remove it from the heat until it cools slightly and turn down the heat on the stove top.
Once the lemon apple curd has thickened, turn off the heat and continue to stir for a minute or two until it cools slightly.
Pass the apple curd through a strainer if you like and pour it into a clean jar or bowl. Place plastic wrap over the surface of the apple curd so it doesn't form a skin and put it in the refrigerator to cool completely.