Heat your oven: To 350°F. Line the bottom and sides of a 9×13-inch baking pan with a large piece of parchment paper, extending the four sides and pressing it into the corners. (It will look like a sling).
Prepare the rhubarb: Stir together rhubarb, lemon juice and granulated sugar and set aside.
Make the cake: Beat butter, granulated 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. Beat in sour cream. Sprinkle baking powder, salt, and ginger over the batter and beat thoroughly to disperse into the batter. Add flour and beat just until it disappears.
Make the crumbs: Whisk the brown sugar, salt, and cinnamon into the melted butter until smooth, then add flour all at once. Stir together with a fork, creating large and small crumbs.
Assemble the cake: Dollop cake batter across the bottom of the prepared pan in spoonfuls then use a spatula — offset, if you have one, makes this easiest — to spread the cake into an even, thin layer. Pour the rhubarb mixture, including any liquid accumulated in the bowl (this is essential for cake moisture) over the cake, spreading it into an even layer. Scatter the crumbs evenly over the rhubarb layer.
Bake the cake: In your preheated oven for 50 to 60 minutes. The cake is done when a tester comes out free of the wet cake batter below. It will be golden on top. Cool completely in the pan on a rack.
To serve: Use the parchment paper to transfer the cake from the pan to a cutting board. Cut into 2-inch squares and go ahead and eat the first one standing up. [It’s called a “snacking” cake for a reason.]
Leftovers keep at room temperature for 3 days, and up to one week in the fridge.