Heat a small pan over medium heat and add the coconut oil (or butter).
Add the diced apple, a pinch of salt, and cinnamon. Cook for about 2 minutes, stirring.
Cover, reduce the heat to low, and cook until the apples are soft and fragrant. Let it cool.
Combine vanilla, oats, milk, Greek yogurt, and maple syrup. Stir well.
Divide the oat mixture between the glass jars. Add the cooked apple on top.
Cover and refrigerate overnight (at least 4 hours).
In the morning, stir and add a splash of milk if it’s too thick.
Top with walnuts or granola right before eating for crunch.