Melt the butter in a medium skillet over medium heat.
Add the diced apple, brown sugar, cinnamon, and water. Cook for 4 to 5 minutes, stirring occasionally, until the apples soften and the liquid thickens into a light syrup.
Remove the skillet from the heat and let the apple mixture cool.
In a small mixing bowl, mix the rolled oats, milk, Greek yogurt, maple syrup, vanilla extract, cinnamon, nutmeg, and salt until combined.
Divide the oat mixture evenly between 2 mason jars or airtight containers.
Spoon the cooled apple mixture evenly over the oats in each jar.
Cover the jars with lids and refrigerate for at least 4 hours or overnight, until the oats soften and absorb the liquid.
Stir gently before serving, or leave the apple topping layered on top.
Notes
Tips and Tricks
Choose the Right Apple – Firm apples such as Honeycrisp, Gala, or Granny Smith hold their shape nicely after cooking.Adjust the Thickness – If your oats seem too thick after chilling, stir in a splash of milk before eating.Keep the Topping Separate – For more distinct layers, store the cooked apples in a separate airtight container and add them to the oats just before serving.Make It Less Sweet – Reduce the maple syrup slightly if you prefer a breakfast that's not quite as sweet. The apple topping already provides sweetness.Try Different Textures – Stir the apples into the oatmeal for cinnamon apple flavor throughout, or leave them on top for a more noticeable contrast.
Storing
Refrigerator – Store the overnight oats in covered mason jars or airtight containers in the refrigerator for up to 4 days.Apple Topping – Keep the cooked apples refrigerated in a separate airtight container for up to 4 days if you prefer to add them just before serving.Freezing – Freezing isn't recommended because the yogurt and cooked apples can change texture after thawing. These oats are best prepared fresh and refrigerated.