Preheat a water bath to 84°C.
Combine the oats, apple, water, cinnamon, and salt in a pint 450ml jar. Screw the lid on the jar until it is finger tight. This will allow some air to escape during the sous vide process and help prevent breakage.
Shake the jar, then carefully place it in the water bath. Cook the porridge for 30 to 45 minutes, until the water is absorbed and the porridge is cooked through, shaking the jar once or twice during the process.
Spoon the porridge into a bowl. Drizzle with maple syrup, then top with almond slivers and apple slices.