Whisk the apple cider vinegar into the nondairy milk.
In the bowl of a stand mixer with the dough hook attached, or in a large bowl if kneading by hand, place the sourdough starter. Add the sugar, warm water and almond milk mixed with vinegar and mix together on low speed or by hand.
Add the flours and xanthan gum directly into the bowl, one by one, kneading a little as you go along so they are thoroughly incorporated. Knead for a couple of minutes until everything is well incorporated and you have a tacky, slightly sticky dough. If your dough looks dry, drizzle in a little more water and knead.
Drizzle in the oil and knead for a couple more minutes until the oil is absorbed.
Remove the dough to a countertop and knead slightly before shaping into a loaf. You might want to oil your hands lightly before handling the dough. It should be slightly tacky but not so sticky that you can't lift it off the surface with just a little dough sticking to it.
Grease a standard loaf pan, either with oil or cooking spray
Place the shaped loaf inside the loaf pan, sprinkle the top with the oats if using, cover loosely with a kitchen towel (or a shower cap), and place overnight or about 10 hours in a warm place, like the oven with the pilot light on.
About a half hour before baking, preheat the oven to 500 degrees Fahrenheit.
Place the bread in the oven and bake 30 minutes
After 30 minutes, turn the heat down to 450 degrees. Bake another 20 minutes.
Remove the loaf pan to a rack and let it stand at least 30 minutes or until cool enough to handle. The bread should unmold quite easily but if it doesn't, run a knife along the sides of the pot to help it along. Let the bread continue to cool on a rack, then slice and serve.