In the evening: In a bowl add the starter, water, sugar, and oil and stir together with a fork,
Add the warm water and buttermilk and stir to combine.
Add the flours and the salt on top. Use your fingers to swirl in the salt to the flour then mix the flour in to the liquids. If the dough is very shaggy or there's a lot of dry flour in the bottom of the bowl, add a few tablespoons of warm water to bring it together.
Let the dough sit for 30 minutes, then use your hands to gently knead the bowl 6 or 8 times in the bowl until the dough is in a smoother ball.
Cover with plastic wrap and leave to rise for 12 to 16 hours/overnight.
In the morning, turn out the dough on to a countertop. Try not to flour it at all if you can or just very lightly.
Use your fingers to press the dough out into a rough rectangle about as wide as your loaf pan.
Roll the dough up like a sausage and place the seam on the bottom.
Cup your hands on either side of the back of the dough and drag it toward yourself, tightening the surface of the bread.
Flour your hands as needed to prevent sticking and drag the dough three or four times, then move it to a greased and parchment paper lined 9x5 loaf pan to rise.
Cover with a damp towel or well oiled plastic wrap and allow to rise until the dough is 1 to 2 inches above the top of the loaf pan--usually 6 to 10 hours depending on how warm the room is. Re-wet the towel if it starts to dry out or it will stick to your bread!
Preheat the oven to 400 and bake the loaf on the center rack close to the door, for 40 minutes; rotating it after 20 minutes. Tent with foil after 30 minutes to prevent over browning.
Check your bread's temperature if needed. It should be 180-200 in the center. Let cool completely before slicing to prevent a gummy texture.