In a small sauce pan over low heat bring buttermilk to between 105°F and 110°F. Remove from heat, add honey and yeast. Allow to sit for 5 minutes.
While the yeast is proofing, add whole wheat flour, rye flour, and 2 cups of bread flour, brown sugar, and salt to a stand mixer. Using the paddle attachment mix flour, sugar, and salt until combined.
With mixer on low, slowly add the yeast mixture and Guinness to flour mixture. Continue mixing, using paddle attachment, until combined. Scrape down the sides of the bowl.
Switch to the bread hook and mix until dough forms around the hook and pulls away from the sides of the bowl. You may need to add the additional ½ cup bread flour at this point. Mix for approximately 3-5 minutes. This step is important as you want to ensure the gluten is developed. When finished your dough should be smooth and slightly sticky. If it is not, continue mixing with the dough hook until this is achieved. If necessary, add additional flour a little at a time until desired consistency is reached.
Cover mixing bowl with towel and allow to rise in a warm area for one hour.
While the dough is rising grease a 9 x 5 loaf pan using butter or coconut oil and set aside.
After the initial rise, remove the dough from the mixing bowl and deflate the dough by punching it down. Lightly knead the dough and shape into a log.
Place dough in prepared loaf pan, cover and set aside for an additional 45 minutes to 1 hour until puffed and slightly risen.
While the dough is proofing, preheat oven to 350°F.
After the second rise sprinkle, score top of bread with a bread lame or sharp knife and sprinkle with oats, if using. Bake for 40-45 minutes in a preheated oven. Bread is done when internal temperatures reads 195°F.
Allow bread to cool for 10 minutes on a wire rack before removing from pan.