Mix together the flour, walnuts, mixed herbs, bicarbonate of soda, xanthan gum and salt in a large bowl, then make a well in the centre. Whisk together the rice milk and rapeseed oil in a jug, then pour into the well and stir in with a wooden spoon until a soft, slightly sticky dough is formed.
Turn the dough out on to a lightly floured surface and pat into a 18 cm (7 inch) round. Place on to a baking sheet. Scatter over the extra walnuts and gently press to adhere to the dough. Make a deep cross in the dough with a sharp knife, then leave to stand in a warm place for 30 minutes.
Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 40–45 minutes until the bread is crisp on the outside and cooked through – the base should sound hollow when tapped with the fingertips. Turn out on to a wire rack to cool before slicing thickly to serve.