Preheat the oven to 425°F (215°C). Line a baking tray with parchment paper and set aside.
Mix together the flours, salt, and baking soda in a large bowl.
Add the butter and rub it into the flour mixture with your fingertips until it resembles course bread crumbs.
In a separate jug, whisk the egg and buttermilk together (see note on how to make Buttermilk below).
Make a well in the center of the dry ingredients and pour in the liquid, hold a little back.
Using an open hand bring the flour and liquid together to a loose dough. The dough should be quite soft, but not too sticky. You will know then if it needs more of the liquids. (Flour in different places reacts differently to added liquid).
Turn onto a floured work surface and gently bring the dough together into a round, about 1 1/2 inches (4cm) thick and 8 inches by 8 inches.
Place on the baking sheet and score the bread by blessing it with a deep cross on top (see video). Poke a hole in the 4 corners of the bread to release the fairies and stop them from cursing your beautiful bread.
Glaze the bread with the leftover bit of buttermilk in your jug and scatter the oats on top.
Bake for 15 minutes, then turn down the oven to 400°F (200°C) and bake for 30 minutes more. When done, the loaf will sound slightly hollow when tapped on the bottom.
Remove from the baking sheet and place on a wire rack to cool.
Once cooled, slice the bread and enjoy with butter. Store leftovers covered at room temperature for up to 3 days.