Place the whole wheat flour and the white flour in a large mixing bowl. Add the cold tallow and cut it into the flour, using either a pastry cutter or a knife, until the tallow pieces are the size of peas.
Add the sourdough starter and 1 cup of milk and combine with a spoon or a dough whisk.
Add the remaining ½ cup milk a little at a time (you may not need all of it) until the sides of the bowl are mostly clean and the dough starts to come together.
Using your hands, very briefly knead the dough together until it forms a rough ball.
Cover and leave the dough to ferment in a cool place (or in the refrigerator if your house is very warm) for 6-8 hours. It is essential that the dough remains cool, otherwise the biscuits won't rise well and will be rather dense.
When the fermenting time is up, preheat your oven to 220C/420F.
Tip the dough onto the counter. Flatten it into a roughly 2 inch thick circle. Sprinkle the baking powder, baking soda and salt evenly over the dough.
Fold the dough about 8-10 times until the baking powder, baking soda and salt are incorporated. This will take some muscle power. Fold the dough in half, press it down, turn the dough 90 degrees, fold in half, press again, turn 90 degrees - repeat this sequence until everything is incorporated.
Divide the dough in half and place one half of the dough on a lightly floured counter. Roll the dough out into 1 inch thick circle. With a round cookie cutter cut out the biscuits and place them on an ungreased baking tray. Repeat with the other half of the dough.
Bake the biscuits for 10-12 minutes until well risen and golden on top. Let them cool a little on cookie racks before serving.
Serve with scrambled eggs for breakfast, with sandwich toppings for lunch, or alongside a soup or a stew for dinner.