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Mix the plant milk and cider vinegar together in a jug, then set aside for 30 mins. Heat the oven to 220C/200C fan/gas 7 and line a baking sheet with baking parchment.
Combine both flours, 1 tsp salt and baking soda in a large bowl, then pour over the plant-milk mixture and stir to combine. Bring together into a dough using your hands, then form into a ball. If the ball is very sticky, lightly flour your hands to make it easier to handle. Place the dough ball onto the prepared baking sheet and flatten slightly into a thick disc using your hands. Cut a deep cross into the centre using a serrated knife, then sprinkle over the oats and bake for 35-40 mins until the base sounds hollow when tapped. Leave on the baking sheet for 5 mins, then transfer to a wire rack to cool completely. Slice and serve.