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Heat oven to 140C/120C fan/gas 1 and line 2 baking trays with baking parchment. Mix the dry ingredients in a large bowl. Beat the egg with 250ml water in a jug, then pour into the dry mixture. Combine with a wooden spoon, then transfer to a lightly floured work surface and knead until you have a smooth, firm dough.
Roll the dough out as thinly as possible and cut into squares, about 7cm. Transfer the squares to your baking trays. Bake for 45 mins, then remove the trays from the oven. Flip each cracker over on the tray and return to the oven, swapping over the shelves, for a further 45 mins. Once cooked, remove from the oven and transfer to a wire rack to cool. Store in a sealed container for up to 2 weeks.