Preheat the oven to 220°C (425°F/Gas 7). Brush the inside of the tin generously with olive oil.
Sift the dry ingredients into a large bowl and make a well in the centre. Pour most of the buttermilk in at once and, using one hand with your fingers outstretched like a claw, mix in the flour from the sides of bowl, adding more buttermilk if necessary.
The dough should be softish, but not too wet and sticky.
When it all comes together, turn it out onto a floured board and roll it out so that it will fit into the swiss roll tin.
Make dimples all over with your fingertips, on the top of the dough.
Next peel the onions and cut first into 6 wedges lengthways, then half crossways. Place these chunks on the raw dough, six across and eight down, next to the half of the onion wedges place a pitted olive and a small sprig of rosemary.
Then drizzle over the olive oil and sprinkle with sea salt.
Bake in the oven for about 30 minutes or until the bread is nice and golden on the top and bottom.
If the bread is a good golden colour and you don’t want it to darken any more during cooking, reduce the oven temperature to 200’C/400’F/Gas mark 6 and continue to bake for the specified time.
When the focaccia is cooked but still hot, drizzle just a little more olive oil over the top and allow to cool slightly before serving.