Add the mashed banana, water, oats, and peanut butter to a small pot over medium heat. Bring to a simmer and cook for about 5 minutes, giving a stir every now and then until the oats are tender and most of the water is absorbed.
Divide among 2 bowls and top with fruit if desired and an extra drizzle of peanut butter.
Notes
Nut-free: to make nut-free make sure you use a seed butter such as pumpkin seed butter or sunflower seed butter, or you can use wowbutter.