Preheat oven to 400 F. Grease your 12-cup muffin tin, (I do this using vegetable oil and a paper towel).
In a medium bowl whisk together flour, sugar, oats, baking soda, baking powder, and salt. Set aside.
In a large bowl beat the egg. Stir in the milk, vanilla extract and oil. Add in the mashed banana, and evenly combine.
Gently fold in the flour mixture into the banana mixture, scraping down sides of bowl. Once just combined, gently fold in the chopped nuts.
Divide batter evenly among the 12-cup muffin tin (using a cookie scoop works great for mess-free dividing!) Optional: top each muffin with a sprinkling of oats and chopped nuts. Bake for 15-20 min until toothpick comes out clean.
In a small bowl whisk together all the ingredients until smooth. If glaze is too runny, add more icing sugar; if glaze is too thick, add more milk/water. Drizzle over cooled muffins.