If using fresh milled flour, mill your berries on the fine setting of your grain mill.
In a large mixing bowl, add the flour and oil. Mix until it looks crumbly. You can use a pastry cutter or pastry blender for this job.
Add in the milk and sourdough starter and mix until completely combined with no dry bits of flour remaining.
Cover the bowl with plastic wrap and let the dough ferment either on the counter or in the fridge overnight.
**If fermenting in the fridge, take the dough out for a few hours before proceeding with shaping and baking**
Preheat the oven to 425 degrees F.
Before cutting the biscuits out and baking, make some dimples in the dough with your finger tips.
In a small bowl, add the salt, baking soda, and baking powder and mix well.
Pour over the dimpled dough. Knead the dough for about 1 minute until this mixture is fully combined with the dough.
Turn the dough onto a lightly floured surface and use a rolling pin to roll out the dough into a rectangular shape.
Fold the dough in half and roll out again. A bench scraper helps with this.
Continue this rolling and folding 3-4 times before cutting out the biscuits.
On the last fold, gently pat the dough to 1 1/2 inch thickness and use your biscuit cutter or a large mouth mason lid to cut out the biscuits.
Spray a pan with oil and place the biscuits on the pan.
Brush the top of the dough with a little butter for a more golden brown biscuit.
Bake in the preheated oven for 10-12 minutes until lightly golden brown.
Cool completely on a cooling rack before storing.