Prep the apple. Core and dice one medium apple into small pieces.
Smaller chunks soften more overnight and distribute better.
Mix the dry base. In a jar or container, add 1/2 cup rolled oats, 1 tablespoon chia seeds, 1/2 to 1 teaspoon cinnamon, and a tiny pinch of salt. Stir to combine so the chia doesn’t clump later.
Add the liquids. Pour in 1/2 cup milk and 1/4 cup Greek yogurt. Add 1 to 2 teaspoons maple syrup (or honey) and 1/2 teaspoon vanilla extract.
Stir well until everything looks evenly mixed.
Fold in the apple. Stir in the diced apple. If you like extra texture, save some apple to add fresh in the morning.
Adjust consistency. If you prefer looser oats, add a splash more milk. If you like it thicker, keep it as is.
The chia will thicken it a lot overnight.
Chill overnight. Cover the container and refrigerate for at least 4 hours, ideally 8 to 12 hours. The oats and chia will soak up the liquid and soften.
Finish and serve. In the morning, give it a good stir. Add any toppings you like—chopped walnuts or pecans, a dollop of yogurt, a sprinkle of cinnamon, a drizzle of maple syrup, or a spoonful of almond butter.
Enjoy cold or warm. Eat straight from the fridge, or warm it gently on the stove or in the microwave with a splash of milk until just heated through.