Line a muffin tin with paper muffin cups.
Preheat the oven to 350 degrees F. Set a rack in the middle of the oven.
Sift together the flour, baking powder, baking soda and salt. Set aside.
Using a stand mixer or hand held beaters, cream the butter and sugar for 3 minutes. Add the beaten egg very slowly to incorporate. Stop and scrape the bowl a few times. Set the speed on low and mix in half the flour mixture till blended. Stop and scrape the bowl. Add the sour cream and mix thoroughly. Add the remaining flour mixture. Stop and scrape the bowl to assure it is evenly mixed, increasing the speed and blending each addition till smooth and creamy. On low speed, add the water. Increase the speed at the end for about 30 seconds. Fold in one cup of the blueberries with a rubber spatula.
For the streusel, toss together the brown sugar, flour, cinnamon, salt, oats and chopped nuts. Work the butter through using your hands till it resembles coarse crumbs.
(You can make the batter up to this point, wrap tightly in plastic wrap and store refrigerated overnight. Let sit at room temperature for about 30 minutes before baking. The streusel can be made in advance and stored at room temperature, sprinkled over the cake just when ready to bake).
Scoop the batter evenly in the prepared muffin tins, top with the remaining 1/2 cup berries and sprinkle with the streusel.
Bake for about 25 -30 minutes or until a wire cake tester pressed in the center comes out clean. Serve warm or at room temperature. Makes about 18 muffins.
Recipe by Mani Naill.