Boil water and stir in steel-cut oats. Reduce heat to medium and let simmer for 15-20 minutes. In the last 5 minutes, stir in half of the banana slices and cook until bananas start to soften (I recommending adding the banana right before serving, so if you're making a large batch to save for the week, remove 3/4 of the oatmeal, and only stir the banana into one portion of the oatmeal)