Peel and slice the banana in half. Mash one half of the banana with a fork until it’s broken down. Slice the other half into 1/4″ slices.
Place the oats in a microwave safe bowl, add the water, and stir to combine. Cook the oats in the microwave for 1 minute. Add the mashed banana, maple syrup, and milk and stir to combine. Top the oatmeal with the sliced bananas, walnuts, and sprinkle the nutmeg over the top.