Preheat the oven to 220c fan.
Grease your tin with a wash of neutral oil, making sure to get into all four corners.
Combine the flours, oats, bicarbonate of soda and salt in a large bowl.
In a jug, whisk the buttermilk, treacle and stout until the treacle has dissolved.
Add in the coarsely grated butter to the dry ingredients.
Create a little well in the dry ingredients then pour in your wet ingredients.
Gently squelch the wet and dry together with one hand until there are no visible pockets of flour.
Scrape down the sides of the bowl and lift the wet dough out into your prepared loaf tin.
Smooth the surface of the dough and sprinkle the top with oats.
Using a sharp knife, create a shallow line in the center of the dough.
Bake for 15 mins, then lower the temperature to 180c and continue baking for 30 mins.
Remove from the oven and allow to cool on a wire rack.
Buttermilk is a sour liquid, originally a byproduct leftover after churning cream into butter. Although these days most shop-bought buttermilks are milks cultured with lactic acid bacteria. If you can’t find it you can make it yourself by mixing 1 tablespoon of lemon juice (or vinegar) into 250ml whole milk and leave to stand for 15-20 minutes. It won’t be as thick as traditional buttermilk but it’s a close enough substitution.
