Place all the ingredients in the bowl of a food processor and blend until it looks like breadcrumbs. Set aside in the fridge to keep cold until ready to use. *See note.
Preheat the oven to 325f and grease and line the base and sides of a 9 inch spring form cake pan.
Whisk the flour, baking powder, baking soda and salt together in a medium bowl.
Place the butter and sugar in a stand mixer bowl fitted with the paddle attachment. Mix the butter and sugar on medium high until very light and fluffy.
Scrape down the sides and bottom of the bowl and add the eggs and vanilla and mix for another 30 seconds until just combined.
Add the flour mixture and sour cream alternately. Add them in three additions beginning and ending with the flour. Pour the butter into the cake pan and cover the and refrigerate for about 20 minutes to firm it up.
Place the sliced nectarines and blueberries on top and sprinkle with about ½ cup of the streusel topping.
Bake for 35-40 minutes, or until the cake is golden on top and the blueberries have burst and the top springs back when touched lightly.
This streusel recipe makes more than required. Freeze the remaining and sprinkle on top of cakes, cookies and fruit crisps.