Put all ingredienst for the pastry in a bowl & mix & knead w/ your hands until all is combined.
Form a plate or a ball, wrap in cling foil & put it in the fridge (fresh section) for about 30 min.
In the meantime clean & trim the rhubarb; cut it in pieces of about 4-5 cm & cut each piece lengthwide once or twice.
Add all the other ingredients & mix thoroughly.
Preheat the oven to 180° C w/ fan. Place a baking tray in the oven.
Flatten the pastry w/ a rolling pin between layers of cling foil until alomost round w/ a diameter of about 30 cm.
Remove the clingfoil on top & add a sheet of baking paper. Turn over & remove the rest of the clingfoil.
Add the rhubarb mess in the middle & leave a rim of 4-5 cm.
Fold in the pastry.
Whip the egg & brush the pastry generously w/ the egg wash. Add some vanilla sugar on top.
Move the galette on the baking parchment carefully onto the hot baking tray & let it bake for about 30 min.