Combine all the dry ingredients in a bowl. Add the rapeseed oil and boiling water, and mix well. Let the mixture sit for about 15 minutes to swell.
Spread the mixture thinly and evenly over the prepared baking sheet.
Bake in the middle of the oven for 1–1½ hours, until crisp.
Remove the crispbread from the oven, and transfer the parchment paper with the crispbread onto a cooling rack. Let cool completely before breaking into pieces.