In a medium-sized mixing bowl, combine the rolled oats, almond milk, diced apple, ground cinnamon, maple syrup, vanilla extract, Greek yogurt (if using), and a pinch of salt. Stir until all the ingredients are well incorporated.
Taste the mixture and adjust sweetness by adding more maple syrup if desired.
Divide the mixture evenly into two mason jars or airtight containers.
Seal the jars/containers tightly and place them in the refrigerator overnight (or for at least 4 hours) to allow the oats to soak and soften.
In the morning, give the oats a good stir. If they seem too thick, you can add a splash of almond milk to reach your desired consistency.
Top with the chopped walnuts for added texture. You can also sprinkle a bit more cinnamon on top if you like.
Notes
Serve the oats directly in the jars for a rustic look, and garnish with a slice of fresh apple or a sprinkle of granola on top for added visual appeal. Enjoy cold!