In a medium bowl, mash the banana (if using) with a fork or add the pumpkin. Whisk in the egg, the maple syrup or honey, and baking powder, and then stir in the oats until thoroughly combined.
Transfer the mixture to a lightly-greased microwave or oven-safe ramekin, mug, or bowl (I’ve found there’s no need to grease when using microwave), and smooth the top with the back of a spoon or spatula. (I’ve also used mini loaf pans and 8- and 16-ounce ramekins; note cooking time variation below.)
Bake in an oven preheated to 350 degrees F for approximately 15 minutes for a 2-cup ramekin or mini loaf pan, or 18 minutes for a 1-cup ramekin. (If you happen to stir in fresh or frozen fruit, the added moisture will necessitate an extra minute or two of bake time. If you do this, I recommend no more than 1/4 – 1/3 cup of fruit. Too much moisture will create a mushy end result.)
Conversely, microwave for approximately 1 minute and 30 seconds, or until the top of the oatmeal is slightly firm to the touch. (If microwaved too long, the oatmeal will become tough. Check after one minute until you know how long this takes in your microwave, extending the time as needed. Time will also vary based on proportions of baking dish or mug used.)
Top as desired and enjoy a speedy, whole-grain, protein-rich breakfast!