Prep the apple: Core and finely dice 1 medium apple.
If you’re prepping several jars, dice multiple apples. Toss with a squeeze of lemon juice if you want to prevent browning.
Mix the base: In a bowl or jar, add 1/2 cup rolled oats, 1/2 cup milk, 1/3 cup Greek yogurt, 1–2 teaspoons chia seeds, 1–2 teaspoons maple syrup, 1/2 teaspoon vanilla, 1/2 teaspoon cinnamon, and a small pinch of salt.
Add the apple: Stir in about 1/2 cup diced apple. Press the mixture down so oats are submerged in liquid for even soaking.
Adjust thickness: If you like thicker oats, add another teaspoon of chia.
For looser oats, splash in more milk.
Chill: Cover and refrigerate at least 4 hours, preferably overnight, so the oats soften and flavors meld.
Serve: In the morning, stir well. Add a drizzle of maple syrup if needed, more cinnamon, and your favorite toppings like chopped walnuts, raisins, or a spoonful of almond butter.
Make it warm (optional): Heat gently in the microwave for 30–60 seconds, stirring halfway, until just warm. Add a splash of milk if it thickens too much.