In a large bowl or a bowl of a stand mixer, combine the starter, water, whole milk, and honey. Next, add the butter, oats, and flours. Mix these until just combined. It should be a shaggy dough. Let the dough rest for 20 minutes. This is the autolyse.
While the bread is resting measure out your salt and set aside. After the 20-minute rest, add the salt and knead the dough by hand or with a stand mixer fitted with a dough hook attachment for 8 minutes. Scrape the sides of the bowl as needed.
Next, take the dough and transfer it to a large, greased bowl, covered with a damp tea towel or plastic wrap, for the dough’s first rise. Let rise for about 3-4 hours depending on the temperature of the room. The dough should be risen and puffy but not necessarily double in size.
After the first dough rise, shape the bread dough and place it in a greased loaf pan. Cover with a damp tea towel and for best results let rise until the bread is about an inch above the pan edge. This can vary depending on the temperature in the room, but it is roughly 4-8 hours. This second rise is also when most of the bulk fermentation happens. Place it in a warm place like the oven with just the light on to speed up this process.