In a medium bowl, whisk to combine the flour, chopped pistachios, sugar, cinnamon, salt and the zest. Use a pastry cutter, two forks or your fingers to cut the cold butter into the flour and sugar mixture until it forms small pebble sized crumbs. Some can be large and some smaller. Place in the fridge to chill until needed. 

Preheat oven to 350 F. Lightly butter or spray then line a 9×9″ pan with parchment paper. Leave some excess parchment paper hanging over two sides to make it easier to remove the bars once baked and cooled.

In a large bowl using a hand held mixer or in the bowl of an electric mixer fitted with the paddle attachment, cream together the butter and sugars.

Mix in the eggs one at a time and mix until combined. Mix in the vanilla and lime zest. Scrape down the sides of the bowl.

Using a rubber spatula, mix in half of the sour cream then stir in the half of the flour mixture and then the rest of the sour cream followed by the rest of the flour mixture until completely combined being careful not to overmix.

Stir in the rhubarb (making sure it's evenly distributed). Spread the batter into the prepared pan. Top with the crumble.

Bake 33-37  minutes until a toothpick comes out clean or with just a few crumbs when inserted in the center. If the crumble starts to get too browned before the cake's done, tent the top of the cake loosely with aluminum foil. Allow to cool to room temperature before serving.