Mash and prep: In a small bowl, mash half the banana until mostly smooth. Slice the remaining half for topping.
Combine in a pot: Add oats, milk, mashed banana, cinnamon, vanilla, chia or flax (if using), and a pinch of salt to a small saucepan.
Stir to combine.
Cook gently: Bring to a light simmer over medium heat. Reduce to medium-low and cook, stirring often, for 4–6 minutes until thick and creamy.
Add protein: Remove from heat. If using protein powder, whisk it in slowly while the oats are warm but not boiling to avoid clumping.
If using Greek yogurt, fold it in gently off heat for a silky texture.
Adjust texture: If the oats are too thick, splash in a bit more milk and stir until it reaches your preferred creaminess.
Top and serve: Transfer to a bowl. Add sliced banana, a spoonful of nut butter, and a sprinkle of nuts. Drizzle a little maple syrup if you want extra sweetness.