Coat the bottom and sides of a 9×13-inch baking pan with butter, then line the bottom with parchment paper, extending the lengths up two sides
Combine sliced rhubarb, lemon juice and 2/3 cup sugar and set aside
Beat 4 oz of butter, remaining sugar and lemon zest with an electric mixer until light and fluffy
Add eggs, one at at time, scraping down the sides after each addition
Whisk together 1 1/3 cups of flour, baking powder, 3/4 teaspoon table salt and ground ginger together in a small bowl. Add one-third of this mixture to the batter, mixing until just combined. Continue, adding half the creme fraiche, the second third of the flour mixture, the remaining creme fraiche, and then the remaining flour mixture, mixing between each addition until just combined.
Pour the batter into a pan and spread evenly into a thin layer.
Strain rhubarb mixture and discard liquid (or reserve for another use, such as a rhubarb cocktail!). Pour the rhubarb mixture over the cake, spreading it evenly over the batter.
Melt 4 tablespoons of butter.
Make the crumb mixture by whisking 1 cup of all-purpose flour, 1/4 cup of brown sugar, 1/8 teaspoon of table salt and 1/4 teaspoon of cinnamon together, then stirring in the melted butter with a spoon. Scatter evenly over rhubarb layer.
Bake cake in preheated oven for 50 to 60 minutes. The cake is done when a tester comes out free of batter. It will be golden on top. Cool completely in the pan.