Preheat oven to 350° F. Coat the bottom and sides of a 9x13-inch baking pan with butter or a nonstick cooking spray, and then line the bottom with parchment paper, extending the lengths up two sides.
Stir together rhubarb, lemon juice, and 2/3 cup sugar and set aside.
Beat butter, remaining sugar, and lemon zest with an electric mixer until light and fluffy. Add eggs, one at a time, scraping down the sides after each addition.
Whisk together flour, baking powder, ¾ tsp. salt and ground ginger together in a small bowl. Add 1/3 of this mixture to the batter, mixing until just combined. Continue, adding half the sour cream, the second third of the flour mixture, the remaining sour cream, and then the remaining flour mixture, mixing between each addition until just combined.
Dollop batter over prepared pan, then use a spatula to spread the cake into an even, thin layer. Pour the rhubarb mixture over the cake, spreading it into an even layer.
For the Crumb
Whisk the flour, brown sugar, salt, and cinnamon together. Stir in the melted butter with a spoon or fork. Scatter evenly over the rhubarb layer. Bake the cake in the preheated oven for 50-60 minute. The cake is done when a tester comes out clean; the cake will be golden on top. Cool completely in the pan on a rack.
Cut the two exposed sides of the cake free of the pan, and use the parchment paper to remove the cake from the pan. Cut into 2-inch squares. Serve and enjoy!
Recipe Note
Cake keeps at room temperature for a few days, but you can also store it in the refrigerator tightly covered.
