Heat milk in a microwave-safe container in 30-45 second increments until it is warm to the touch but not scalding. Pour into bowl of stand mixer.
Add yeast to warmed milk. Allow to bloom while you gather your other ingredients.
Mix in sugar.
Add sourdough starter, avocado oil, baking soda, and salt. Mix until combined.
Mix in flour, one cup at a time. Once all flour has been added, allow the mixer to knead the dough at medium speed for approximately 5 minutes. The dough should be slightly sticky, but should pull cleanly from the dough hook and the sides of the bowl. If it seems too sticky, add a tablespoon of flour and mix in. Repeat until dough is the appropriate texture, but do not add more flour than necessary.
Transfer kneaded dough to a large bowl that has been greased. Cover. Place in a warm place to rise for 30 minutes, or until dough has doubled in size.
Once your dough has doubled, split in half. Place each half in a greased loaf pan.
Optional: score the top of each loaf using a serrated knife. Using a pastry brush, coat the top of each loaf with the beaten egg.
Allow the loaves to rise in a warm place for another 20-30 minutes. While the dough is rising, preheat your oven to 400 degrees F.
Bake the loaves on the middle rack of the oven for 25-30 minutes. The loaves should sound hollow when you tap their top, and their tops should have a golden sheen when they are done. Smaller loaf pans will require less time, and their corners may burn if baked for longer durations.
Cool loaves in their pans for 10 minutes before carefully removing. Allow to cool for another 30 minutes before slicing and serving.