Add both flours to a large bowl, along with vanilla sugar, salt, and brown sugar. Mix and add coconut oil which should be about room temperature a.k.a. a little soft. Knead well to form a soft ball of dough. Leave the ball in the bowl until you need to use it again.This dough is going to be very soft so treat it gently both while rolling it out and folding it. It tends to crack and tear but it also results in a very soft and crumbly crust when baked. Next up: rhubarb. Peel it and cut it into 1 cm pieces. Put it into a larger bowl and add brown sugar, blackberries, and cardamom. Mix well.
Place a piece of parchment paper on your working surface. The paper should be big enough to fit a large baking sheet. Flour the parchment paper, place a ball of dough on it and flour the dough, too. Then place another piece of parchment paper on top of the dough. Roll out the dough placed between the two sheets of parchment paper. Roll it into a large circle. This way of rolling out the dough is fuss-free and requires no cleaning afterward. Move the dough along with two parchment papers to a large baking sheet. Remove the top piece of paper. Place your rhubarb filling in the middle of the dough, leaving about 2-3 cm of an edge around the filling. Fold the edge towards the middle of the galette.
Use the juice that leaks out and brush the folded edges with it. Sprinkle it with some brown sugar.
Bake the galette for 30 minutes at 180°C. Once baked, leave it for a few minutes so it cools down a bit.
Serve it hot or cold, but it holds together better when cold. You can add some whipped cream or ice cream et voilá! Enjoy!