In a large mixing bowl, combine the rolled oats, almond milk, apple cider vinegar, maple syrup, ground cinnamon, vanilla extract, chia seeds, and a pinch of salt. Stir well to combine.
Fold in the diced apple, ensuring it's evenly distributed throughout the mixture.
If you're using walnuts, add them in now for an added crunch and mix well.
Divide the mixture into two individual jars or bowls with lids. Seal them tightly.
Refrigerate the jars overnight (or at least for 4 hours) to allow the oats and chia seeds to soak up the liquid and soften.
In the morning, stir the oats to combine, and if you prefer a creamier texture, add a splash more milk.
Optionally, top with additional diced apples, a sprinkle of cinnamon, and a drizzle of maple syrup before serving.
Notes
Serve the oats in clear jars to showcase the layers and colors, and garnish with a sprig of mint or a few apple slices on top for an appealing visual touch.