Starting with active sourdough starter (be sure your starter has been fed within 4-8 hours), combine sourdough starter, water, oil, and salt
Using the dough hook on your stand mixer, mix these ingredients together on low speed until just combined.
Slowly add in the flour, 1 cup at a time, while mixer is running. Once all flour is added, let your mixer run for about a minute. Turn mixer off and let sit for 10 minutes to allow the flour to absorb the liquid.
Turn mixer back on low and let it knead the dough until smooth and elastic (about 4-5 minutes). NOTE: This is a very dense dough and your mixer may struggle to knead it well. Alternatively, you can remove from the mixing bowl and knead by hand.
Add about 1 Tbs avocado oil to a clean bowl, drop dough into the bowl and roll around until all sides are coated with oil. Cover with plastic wrap or a damp tea towel for 8 hours (or overnight).
Turn dough out onto a lightly floured surface. (If you're having problems with your dough sticking to the counter, you can lightly flour a tea towel and continue working on that.)
Using a knife (or pizza cutter!) divide the dough into 12 equal portions.
Roll each portion into a ball, then pinch through the center with your thumb and pointer finger. Gently stretch the dough into a bagel shape. (I found over stretching them was important as the dough tends to want to shrink back. I aimed to have a hole about the size of a 50 cent piece.)
Place bagels onto a cookie sheet lined with a silpat mat (can use parchment paper, however I've had problems with my bagels sticking to them once adding the egg wash).
Cover the bagels with a damp tea towel and let rise for one hour. (Bagels may not appear much larger after an hour, that's OK!)
Preheat the oven to 450℉. Fill a large pot 2/3 full with water and bring to a rolling boil. Add 1 tablespoon baking soda.
Working in batches, drop as many bagels as will fit into your pot without overcrowding and let cook for 30 seconds. Flip each bagel and cook for 30 more seconds. Remove bagels and place on a lightly greased baking sheet. Continue until all bagels have been boiled.
In a small bowl, combine egg and 1 Tbs water to create an egg wash. Brush the bagels with egg wash and add additional toppings, if using. (See our favorites below).
Bake for 15-20 minutes, or until bagels are cooked through and no longer doughy (you may need to rotate the pan halfway through baking).