Prep. Preheat the oven to 375°F (190°C) and put paper liners in the muffin tins. This recipe can make either 12 large muffins or 15 regular-sized muffins.
Make the batter. In a large mixing bowl, beat together the oil, sugar, vanilla, and egg using an electric mixer on medium speed until well mixed. Then, add the mashed bananas and sourdough starter, mixing on low speed until combined. In a separate bowl, whisk together the flour, salt, baking soda, and baking powder. Slowly add the dry ingredients to the wet ingredients, mixing on low speed until just combined. If you like, add walnuts and fold them in.
Make the crumb topping. In a small bowl, whisk together oats, flour, sugar, and salt. Stir in the melted butter until it forms clumps. Add the walnuts if you're using them.
Assemble. Split the batter equally into the muffin cups, filling each about ¾ full. Then, add 1-2 tablespoons of the crumb topping to each muffin.
Bake. Bake for 20 minutes or until a toothpick comes out of a muffin with moist crumbs. Let the muffins cool in the tin for a bit before moving them onto a wire rack to cool completely. You can serve them warm or at room temperature.