In a medium mixing bowl, combine the rolled oats, almond milk, maple syrup, ground cinnamon, nutmeg, and chia seeds. Mix thoroughly to ensure all ingredients are well incorporated.
Gently fold in the diced apple, making sure to distribute it evenly throughout the oat mixture for balance in flavor and texture.
Carefully divide the mixture into two jars or containers, filling each one about three-quarters full to allow for expansion.
Add a generous dollop of Greek yogurt on top of each jar, then sprinkle with chopped walnuts and raisins if you choose to include them for added flavor and crunch.
Seal the jars tightly with their lids, then store them in the refrigerator. Allow them to chill overnight (or at least five hours) so the flavors can meld and the oats can soften.
In the morning, simply grab a jar from the fridge. Stir gently to combine the layers, and enjoy your delicious apple pie overnight oats cold. If you prefer, you can warm it up in the microwave for about one minute.
Notes
Serve in clear glass jars for an appealing presentation. Garnish with cinnamon and apple slices.