Add avocado oil to a cast iron skillet and heat to medium heat. Sauté the onion for 5 minutes, until it begins turning translucent.
Transfer the sausage to the hot skillet and brown for 2 minutes. Flip and brown another 2 minutes, until sausage has a nice golden-brown sear on it. Break up the meat using a spatula and cook another 30 seconds or so. There’s no need to cook the sausage all the way through.
Plug in your slow cooker and add the sauteed onion and browned sausage, along with the rest of the ingredients for the soup. Stir well. Secure the lid on the crock pot and set on high for 4 hours (or low for 6 to 8).