Dissolve the yeast and sugar in the lukewarm water and let it sit for 5-10 minutes until frothy.
Combine all of the dry ingredients in the bowl of a stand mixer. (You can do this by hand if you prefer.) Then add the yeast mixture along with the buttermilk and beer. Attach the dough hook and knead on the bread setting (#2) for 10 minutes. Scoop the batter into a large non-reactive bowl, cover loosely with plastic wrap, and let it sit at room temperature for at least 8 hours, preferably longer, up to 24 hours (the longer the better for the whole grain berries to soften and for heightened sourdough properties also the longer it ferments the more liquid will be absorbed so less chances of a moist center after baking.)
Preheat the oven to 350 degrees F.
Scoop the batter (it will be thick and gooey but not like regular bread dough) into a generously buttered 13x4x4 Pullman loaf pan and smooth the top to even it out. Sprinkle with oats. (Note: Baking this bread in a Pullman loaf pan yields those nice small, square slices of bread. But it also helps ensure that the center of the bread is fully cooked.)
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 thermometer and aim for about 205 degrees F. Let the bread sit in the pan for 5 minutes before removing it, then let it cool completely on a wire rack before slicing it. To get clean, professional slices I highly recommend using a bread slicer.
How to "crack" the whole berries: I use my grain mill to crack them by putting it on the coarsest setting. Another option is if you have a Vitamix and use the dry container - pulse the whole berries a few times until you get coarse pieces of einkorn. A powerful food processor will likewise do the job. Alternatively you can also place the whole berries in a ziplock bag and use a heavy object to pound them to break them up.