Peel your ripe banana and place it into a medium-sized glass jar or airtight container. Use a fork to mash the banana until it reaches a smooth consistency. A few small chunks are fine if you enjoy a bit of texture in your oats.

Add the rolled oats, vanilla protein powder, chia seeds, cinnamon, and a pinch of salt directly into the jar with the mashed banana. Stir these dry components together briefly to ensure the protein powder is distributed evenly before adding the liquid.

Pour in the almond milk and the optional maple syrup. Stir vigorously with a spoon until all the oats are submerged and no clumps of protein powder remain at the bottom. Seal the jar with a lid and place it in the refrigerator for at least 4 hours, though leaving it overnight produces the best texture.

In the morning, remove the lid and give the oats a quick stir. If the mixture seems too thick, splash in a little extra milk to loosen it up. Top with the chopped walnuts and the reserved banana slices before serving.