Preheat the oven to 400°F and grease a baking sheet
In a small bowl or measuring cup, combine the milk and lemon juice (or vinegar) and allow to sit for five minutes or until clumpy.
In a large bowl, sift together both flours, sugar, baking soda, and salt. Use your fingers to break the butter into small pea-sized pieces. Stir them throughout the flour and make a well in the center of the bowl.
Use a fork to whisk in the egg in the soured milk and pour it into the well. Use a spoon to gently combine all the ingredients. Once the ball begins to form, turn the dough out onto a lightly floured surface and use your hands to a pastry scraper to gather the dough until completely combined.
Move the dough to the greased baking sheet and slightly flatten the dough ball. Use a sharp blade to cut an "X" into the top and bake for 45 minutes. Remove from the oven and let cool slightly before serving.
Notes
Always sift the baking soda to prevent sour unwanted clumps from showing up in the bread.
Use a pastry scraper to gather together any loose bits of dough without kneading it.
Lightly pat down the top of the loaf to flatten it. This helps the triangular pieces stick out further from the bread.
For a beautiful flowering loaf that looks like it is blossoming, use a bread lame instead of a kitchen knife to cut the dough.