In a large mixing bowl or a sealed container, combine the chia seeds, milk, maple syrup, vanilla extract, ground cinnamon, and the pinch of salt.

Whisk the mixture thoroughly for about one minute, ensuring there are no clumps of chia seeds stuck together at the bottom.

Let the mixture rest for five minutes, and then stir it vigorously one more time to prevent the seeds from settling too quickly. Cover and refrigerate for a minimum of four hours, or preferably overnight, allowing the pudding to thicken completely.

Preheat your oven to 350°F (175°C). In a small bowl, combine the rolled oats, flour, brown sugar, and cinnamon. Mix these dry ingredients together well.

Cut the cold butter or coconut oil into small cubes and use your fingertips to quickly work the fat into the dry mixture until coarse, pea-sized crumbs form.

Spread the crumble mixture evenly onto a small, parchment-lined baking sheet. Bake for 8 to 12 minutes, or until the topping is golden brown and smells fragrant. Remove from the heat and allow it to cool completely before assembly; this is crucial for maintaining crispness.

Place the diced apples, water or apple juice, brown sugar, cinnamon, and nutmeg in a small saucepan over medium heat. Bring the mixture to a gentle simmer, then reduce the heat to low.

Cook the apples, stirring occasionally, for 8 to 12 minutes, or until the apples have softened significantly but still retain a slight structure. Remove the saucepan from the heat and allow the spiced apples to cool to room temperature before layering.

Retrieve the chilled chia pudding base from the refrigerator and give it a gentle stir. Select your serving containers (mason jars or small bowls).

Spoon a generous layer of the chia pudding into the bottom of each container, filling approximately halfway. Add a substantial layer of the prepared spiced apple filling over the top of the pudding layer.

Finish the dessert by sprinkling a generous portion of the cooled, crispy oat crumble topping over the very top of the pudding.

Serve immediately for the best texture contrast, or cover and refrigerate for up to 24 hours (only add the crumble just before serving to maintain its crunch).