Mix together the milk and lemon juice in a jug and leave to stand for 5 minutes. Stir in the onion and egg.
Put the flours, salt and bicarbonate of soda in a separate bowl and stir in the herbs. Add the butter and rub in with the fingertips until the mixture resembles fine breadcrumbs. Pour the milk mixture into the bowl and mix to a dough, adding a little more milk if the dough feels dry.
Shape the dough into a ball on a lightly floured surface, then place on a greased baking sheet and flatten slightly. Dust generously with extra flour. Using a sharp knife, cut a 2.5 cm (1 inch) deep cross through the top of the dough.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for about 30 minutes until risen and deep golden. The base of the bread should sound hollow when tapped. Transfer to a wire rack to cool.