Preheat the oven to 350°F /180°C ( 160°C fan oven). Line an 8x4 inch (20x10 cm) loaf pan with parchment (baking paper) and set aside.(See note 3)
In a large bowl, mix together the flour, salt and baking soda. Use your fingertips or pastry cutter to incorporate the butter into the flour, until it resembles a breadcrumb like appearance.
Stir the cooled onion and garlic into the flour mixture.
Pour the buttermilk into the flour mixture, using a spatula or wooden spoon to form a sticky dough.
Transfer the dough to the baking pan.
Bake for 35 - 40 minutes, until golden brown.
Remove the baked bread from the oven, but leave in the pan for about 10 minutes before turning out onto a cooling rack.
Allow to cool for about 15 to 20 more minutes before slicing. See note 4