Dice the two onions and put a frying pan on medium heat. Add the butter to the pan and once it has melted throw in the onions.
Cook the onions until they are soft and translucent, about 5 minutes, reduce the heat if they start to brown at all. Scrape the onions into a bowl and allow to cool while you get on with the bread.
Preheat the oven to 200°C.
Add the two flours, salt and bicarb to a large bowl and combine well. Finely chop the rosemary and also add this to the bowl.
Add the buttermilk and cooked onion and gently stir until just combined (over mixing will result in a brick, rather than a loaf of bread).
Once you have a rough dough, turn out onto a lightly floured surface and shape into a round ball (it is totally fine if it looks a bit shaggy).
Move the dough to a lined baking sheet and place in the oven for 35-45 minutes.
The outside of the bread should be a nicely browned and it sounds hollow when you tap it.
Let cool for half an hour or so before you dig in.