Preheat the oven to 350 F.

Butter a 9 inch springform pan thoroughly. Set aside.

Whisk together the flour, baking powder, baking soda, cream of tartar, salt and nutmeg. Set aside.

Beat the butter on moderate speed until creamy. Add the orange and lemon zest and beat an additional 30 seconds. Scrape the sides and bottom of the bowl.

Add the sugar, gradually, while beating the butter on moderate speed. When all of the sugar is added, beat an additional minute, until light and fluffy.

Scrape the sides and bottom of the bowl again.

Add the eggs, one at a time, scraping the bowl as needed. Beat for 30 seconds after adding each egg, incorporating the eggs into the batter. Beat in the vanilla if using.

Remove 1 tablespoon of the flour mixture to a medium sized bowl. Set aside.

On the lowest speed, add the flour mixture and sour cream, alternating, in 4 and 3 additions, respectively. Begin and end with the flour mixture.

Toss the frozen blueberries with the tablespoon of flour. Then gently fold the blueberries into the cake batter by hand.

Scrape the batter into the prepared springform pan. Sprinkle the unbaked crisp topping over the cake evenly.

Bake for 50-60 minutes, until the topping is brown and crisped, and a cake tester inserted into the center of the cake comes out with only a few crumbs or blueberry juice on it.

Let the cake cool in the pan for 10 minutes. Then remove the outer wall of the springform pan and let cool completely.