- Preheat the oven to 375°F.
In a large bowl, mix together the oats, flour, sugar, baking powder, baking soda, cinnamon, ground nutmeg, and salt.
- In a separate smaller bowl, whisk the egg, oil, and vanilla until well combined.
- Pour the wet mixture into the bowl with the dry ingredients and stir until just combined. The mixture will be quite dry at this point.
- Add the mashed bananas and fold them into the batter. Continue folding until all the dry ingredients are fully incorporated.
- Divide the batter evenly into a muffin tin that has been lightly sprayed with nonstick spray or lined with paper liners.
- In another bowl, mix the oats, cinnamon, brown sugar, and melted butter to make the topping. Sprinkle this mixture evenly over the muffin batter.
- Bake in the preheated oven for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.