Preheat oven to 350 degrees F.
Grease a loaf pan.
In a large mixing bowl, add flour, baking powder, baking soda, salt, cinnamon and nutmeg. Stir with a wire whisk to thoroughly combine. Set aside.
In a small bowl of a stand mixer, cream together butter and brown sugar.
Add in the eggs, one at a time, and mix well after each egg is added.
Add vanilla and sour cream and mix well.
Add the butter mixture to the flour mixture and gently mix with spoon to combine.
Gently fold in blueberries, banana and walnuts to batter.
Pour batter into prepared loaf pan.
Combine streusel ingredients in a food processor and pulse to combine until mixture becomes crumbly.
Sprinkle streusel over batter and place in oven. Bake 50-60 minutes or until toothpick inserted in batter comes out clean or with a few crumbs attached. (If toothpick has batter on it, return bread to oven and bake another 5 minutes then re-test.)
Place on rack to cool and store in refrigerator.