In a small bowl, stir together the cornstarch, lemon juice, and water. Add the mixture to the skillet and cook for one more minute, stirring constantly, until thickened and glossy. Remove the skillet from the heat, transfer the filling to a bowl, and let it cool until it is no longer piping hot, about 15 minutes. (If you like, you can put the bowl of filling into an ice bath to cool the apples much faster, in about 5 minutes.)