Rub the cold butter into the flour until crumbly, add a pinch of salt and 4 tbsp of cold water, and knead the dough by hand.
Roll out the dough thinly on a floured table.
Cut the boiled beets into pieces, add salt, sour cream, egg white and chopped herbs, and mix.
Spread the filling over the dough, leaving the edges free.
Fold the edges of the galette in around the filling, brush with egg yolk and sprinkle with flax seeds.
Bake in an oven preheated to 180°C for 30 minutes. Let the finished galette cool a bit.
