For the dough, mix flour, sugar and salt in a large bowl. Add butter pieces and quickly incorporate with fingertips so that the mixture resembles small lumps of bread. Add sour cream and mix everything together with the help of a fork to form a dough.
Form 2 flat balls and place in the refrigerator wrapped in plastic wrap for at least 1 hour.
Remove dough balls from refrigerator and let stand at room temperature for a few minutes.
On a lightly floured surface, roll out with a rolling pin until large enough to cover the bottom of a round cake pan, with an overhang of about 2 1/2 inches.
Place one disk of dough in cake pan. Cut the second disk into strips about 2 inches wideand store in the refrigerator until ready to use.
Preheat oven to 220 °C with a baking sheet on the middle rack.
For the rhubarb filling, place sugar, flour, juniper berries, cinnamon and salt in a stand mixer and mix well. Strain mixture through a sieve into a small bowl, removing all larger pieces of juniper.
In a large bowl, combine rhubarb and lemon juice. Add sugar mixture and stir until well blended.
Pour into pastry lined cake pan. Place strips of dough in a lattice shape on top of filling, pressing firmly at ends. Brush dough with egg-water mixture and sprinkle with sugar.
Bake cake on baking sheet for 15 minutes. Reduce oven temperature to 190 °C and bake for another 35 minutes, until the