Add the apples, cinnamon, maple syrup and a pinch of sea salt to small saucepan over low heat.: Steam rises gently as the apple begins to soften, and you will notice the spice fragrance bloom, which tells you the flavors are starting to marry. The low heat is important so the apple softens evenly rather than caramelizing too fast. A common mistake is turning the heat up, which can scorch the pan and make the fruit unevenly cooked.
Mix to combine then cover and cook for about 10 minutes, stirring occasionally, until the apples are tender.: As they simmer, the apple pieces will relax and the liquid will go glossy, tasting sweeter and deeper. Stirring occasionally ensures nothing sticks and that each piece cooks the same. If you skip stirring, smaller pieces may break down more, leading to a mushier texture than intended.
Remove the lid, increase the heat to medium and cook an additional 2-3 minutes or until the sauce thickens slightly and coats the apples and excess liquid evaporates.: You will hear a soft bubbling as moisture reduces, and the syrup will cling to the fruit, signaling readiness. That concentrated glaze gives each spoonful an intense apple note. A mistake here is overreducing, which can make the mixture dry rather than saucy.
Remove from the heat and set aside while you make the oats.: The warm compote cools slightly and its aroma continues to develop, so set it aside to avoid cooking the oats prematurely. Letting it rest prevents steam from warming the jars too much later. If you rush and layer piping hot apples into jars, condensation can create a watery texture in the overnight oats.
In a small bowl, add the milk, chia seeds, vanilla, cinnamon, nutmeg, and maple syrup.: As you whisk these together, you will notice the warming spices and vanilla perfume the milk, setting the stage for the oats . The chia seeds will absorb liquid and gel overnight, so mixing them now ensures even distribution. If you skip mixing thoroughly, the chia can clump and leave uneven pockets of texture.
Add the oats and mix to combine.: The oats will start to plump as they soak the spiced milk, and the mixture should look homogenized and slightly glossy. Stir until there are no dry patches so the texture is consistent. Not mixing can leave dry oat bits at the bottom, which will remain chalky after chilling.
Add 1/4 of the oat mixture to 2 mason jars (8 oz) or airtight containers (with lids).: As you spoon the first layer of the oats in, you'll feel the weight and notice the creamy surface ready to receive the compote. Layering in jars controls the portions and creates a pleasant distribution of textures later. Avoid overfilling, because the mixture expands slightly and needs space for the apple layer.
Layer each evenly with 1/4 of the apples then top each with an equal amount of the remaining oats and 1 tablespoon walnuts.: The apples provide a sweet, saucy contrast to the creamy layer beneath, and finishing with chopped walnuts adds a crunchy counterpoint. Make sure each jar has equal portions so everyone gets the same experience. A common error is uneven layering, which can make some jars overly sweet and others bland.
Cover and refrigerate overnight.: Cold sets the texture, allowing the oats to plump fully and the chia to thicken, so the jar becomes spoonable and satisfying. Refrigerating also helps the flavors meld and mellow. Do not skip the rest period, or the oats will remain underhydrated and firmer than intended.
Oats may be eaten cold, at room temperature or warm.: When you taste the final jar, you will notice the chilled version is refreshing, room temperature brings out subtler sweetness, and warming slightly revives the apple compote aroma. Heat gently if desired to preserve texture, and avoid microwaving too long which can make the oats overly soft.