Preheat oven to 180 degrees C. Grease 12 muffin cups or line with patty pans.
Place rolled oats in a food processor and process until ground. In a large bowl, combine ground oats with flour, 1 tablespoon rolled oats, chopped walnuts, baking soda, baking powder, cinnamon and salt.
In a separate bowl cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the banana and vanilla. Stir in sour cream. Mix oat/flour mixture into egg mixture. Spoon batter into prepared muffin tin.
Topping: In a small bowl combine rolled oats, brown sugar, cinnamon. Cut or rub in butter until mixture resembles coarse crumbs then stir in chopped walnuts. Generously sprinkle muffins with topping.
Bake in preheated oven for 25 to 30 minutes, until golden brown and a toothpick inserted into a muffin comes out clean.