Stir the yeast and sugar into the lukewarm water and let sit for 10 minutes until the yeast is frothy.
Combine all the dry ingredients in the bowl of a stand mixer. Add the yeast mixture, beer and buttermilk. Stir to combine.
Fit the stand mixer with a dough hook and knead on the bread setting ("2") for 10 minutes. The dough will be very sticky and not remotely malleable.
Scoop the dough into a very large non-metallic bowl with plenty of head space (the dough will bubble up). Cover loosely with plastic wrap and let it rest in a warm place (room temperature) for 24-48 hours, depending on how sour you want the bread to be (be sure to ferment it for at least 24 hours to ensure enough of the liquid is absorbed).
Line a 9x5x3 inch bread loaf pan with parchment paper (SEE NOTE). Preheat the oven to 350 degrees F.
Scoop all of the dough into the lined bread pan, pressing down as needed. (It's a lot of dough but it will fit.) Brush the top with water and sprinkle over evenly with the rolled oats.
Bake on the middle rack for 100-120 minutes or until the center is done. For best and most accurate results use an instant read thermometerand aim for an internal temperature of about 205 degrees F.
Let the loaf cool for 5 minutes before removing it from the pan. Let the loaf cool completely before slicing. Keep stored in an airtight container. To prolong its shelf life I recommend storing it in the refrigerator or freeze part of it if you know it will take you a while to go through it.
* A couple of our readers have mentioned having difficulty peeling the parchment paper from the baked bread. I have never had to do this, but you can lightly spray the paper with some oil. * You can substitute water or more buttermilk for the beer