Preheat the oven to 200°C/180°C fan/Gas Mark 6 and line a baking tray with baking paper.
Cut the top off the garlic bulb to just about reveal the cloves inside, leaving the papery skin on. Place the bulb in a small baking tin, drizzle with the olive oil and bake in the oven for 20–30 minutes until the cloves are soft to the touch. Set aside to cool.
When cool, squeeze the roasted garlic out of each papery casing and into a separate bowl. Mash it up into a paste with a fork or press it through a garlic press, then mix in the buttermilk and whisk together.
In a large bowl, mix together the flours, bicarbonate of soda, salt and pepper, making sure the salt and bicarbonate of soda are well distributed throughout the flour.
Make a well in the middle of the flour mixture and pour in the buttermilk mixture. Use your hands, a spatula or bench scraper to mix the wet ingredients into the dry until a sticky dough forms.
Lightly flour a work surface and tip the dough onto it. Gently roll and fold the dough a couple of times to bring the mixture together. Do not knead – you just want to bring it together.
Shape the dough into a ball and transfer it to the baking tray. Gently press down with your hands to flatten the ball into a disc, about 5cm thick. Liberally dust the top with flour and use a large sharp knife to score the dough with a deep cross dividing it into quarters, about 2cm deep. A ‘sawing’ action can help get a sharp cut.
Bake for 35–40 minutes until golden brown and hollow-sounding when turned upside-down and knocked on the bottom. Once baked, leave to cool on a wire rack.