In a small bowl, add apple pieces and pour in lemon juice; toss gently and set aside.
In a medium bowl, combine flour, oats, cinnamon, ginger, baking soda and salt; whisk until well blended and set aside.
In the bowl of a stand mixer, add brown sugar, butter, and apple sauce. Using the paddle attachment, process on medium speed until the mixture is well blended. Add egg and vanilla extract; process until incorporated.
Add dry ingredients to the egg mixture, reduce the speed to low and mix until just combined, about 30 seconds. Add apples with any accumulated juices and if using, walnuts; process until just mixed, about 15 seconds.
Transfer the dough to a bowl, cover with wax paper making sure it touches the surface and chill for 24 hours.
Preheat oven to 350ºF and line two large baking sheets with parchment paper or silicone mats; set aside.
Take the dough out of the fridge and let it get to “almost” room temperature.
Scoop up 1 ½ tbsp. cookie dough and roll each into a tall ball. Place them on the prepared baking sheets, spacing them evenly; flatten the dough slightly.
Transfer to the preheated oven and bake for 18 to 20 minutes or until lightly golden and set on the edges.
Remove from the heat and allow to cool 5 minutes before transferring them to a wire rack to cool completely.