Preheat oven to 375°F (190°C). Grease a 12-cup muffin tin.
In a large bowl, combine the flour, oats, brown sugar, baking powder, baking soda, and salt. Stir in the walnuts and raisins. In a separate bowl, whisk together the egg, milk, and vegetable oil.
Add the wet ingredients to the dry ingredients and mix until just combined. The batter should be thick and slightly lumpy.
Divide the batter among the muffin cups, filling each about three-quarters full. Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
Let cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Interesting Facts
Muffins are believed to have originated in the United States in the early 19th century.
Oatmeal is a great source of fiber, protein, and healthy fats.
Walnuts are high in antioxidants and are a good source of healthy fats.