In a medium bowl or two 16-oz jars, combine the oats, chia seeds, cinnamon and a pinch of salt; stir briefly so the spices and seeds are evenly distributed.
Add almond milk, Greek yogurt, maple syrup, and vanilla. Whisk or shake vigorously (if using jars, seal and shake) until the mixture is smooth and the yogurt is fully incorporated.
Divide the mixture between jars or portion into two bowls, pressing down gently so there are no large dry pockets of oats.
Cover the containers and refrigerate for at least 6 hours, or preferably overnight (6–10 hours). The chia will absorb liquid and thicken the oats into a creamy texture.
Before serving, open the jars and stir the oats once; if the mixture is too thick, loosen with 1–2 tbsp extra milk until you reach your preferred consistency.
Top each serving with sliced banana, mixed berries and chopped walnuts. Give a final sprinkle of cinnamon if desired and serve cold, or warm briefly in the microwave (20–30 seconds) for a warm breakfast.
Store any leftovers tightly covered in the fridge for up to 3 days; stir before eating and refresh toppings as needed.