1. Add the flour and salt to a large bowl. Cut the butter into small cubes and toss it in the flour to coat it evenly. Use a pastry blender* or knife and cut the butter into pea-sized pieces. Combine cold water and vinegar (you can cool it further down with some ice cubes). Add about half of the water-vinegar mixture to the bowl and mix it in. Gradually add more while working the dough until it starts sticking together – you might not need all the liquid. The dough should not feel wet. Some dry spots in the dough are fine. Wrap the dough in plastic wrap and place it in the fridge for about one hour.

2. Roll out the dough into a rectangle on a lightly floured surface. Fold the long sides towards the center in thirds, one layer over the other, to create a three-layer package. Repeat this process two more times – roll out to a rectangle and fold it towards the center. After the last round of folding, wrap the dough once more in plastic wrap and let it rest in the fridge for at least two hours or (even better) overnight. The dough can be stored in the fridge for up to 3 days.

3. Roll out the dough on a floured surface slightly larger than a 9-inch pie dish*. Place the dough inside the pie dish, press it down, tuck in overlapping dough around the edges and crimp to your liking. Prick the bottom several times with a fork. Place the pie dish in the fridge for about 20-30 minutes.

4. While the dough is resting, add the flour, brown sugar, and melted butter for the streusels to a bowl and mix to combine. Mix in the rolled oats, and place the streusels in the fridge until needed.

5. Preheat the oven to 220°C (425°F). Place a piece of baking parchment on the dough and fill up with baking beans*. Blind bake the pie crust for about 15-16 minutes, take it out, remove the baking beans and baking parchment and bake another 5-6 minutes until the bottom is slightly golden brown – it should not look wet anymore. Take out of the oven and let cool down completely.

6. Reduce the temperature of the oven to 180°C (350°F). Clean the rhubarb and cut it into thin slices. Add it to the cooled pie crust and distribute evenly. Mix the eggs, sugar, cardamom, salt, sour cream, and vanilla extract in a large bowl until well combined. Pour this mixture over the rhubarb, sprinkle the streusels on top, and bake the pie (at reduced heat) for about 50-55 minutes. Take out of the oven and let cool down completely, then place in the fridge for at least 2-3 hours (or overnight) before serving.