Preheat the oven to 400°F (200°C or 180°C for a fan oven).

Brush a 9 x 13 inch (23 x 33 cm) baking tin with olive oil.

Sift the flour, salt and baking soda into a large bowl.

Make a well in the centre.

Pour all but a couple of ounces of the buttermilk into the well.

Using one hand with the fingers outstretched like a claw, mix in the flour from the sides of the bowl until the dough comes together. It should be soft, but not too wet and sticky. If necessary, add the rest of the buttermilk, a bit at a time.

Turn the dough on to a floured bread board and just turn it in the flour, shaping the dough into a rough circle.

Put the dough on the oiled baking tin and roll it out into a circular shape.

Make little holes, or dimples, in the dough with your fingertips.

Pop some rosemary sprigs in the holes and drizzle the focaccia with olive oil.

Sprinkle with the sea salt.

Bake the focaccia for 20 to 30 minutes or until golden brown and cooked through.

Drizzle a little more oil over top, allow to cool slightly and then serve.