Heat the oil in a skillet over medium heat, cook onion until translucent for about 1-2 minutes. (or put the raw chopped onion into a slow cooker to save time).
Add the chicken sausage and cook until browned. Remove from the heat.
Place all the ingredients (except cheese) into a slow cooker.
Cook on low for 7 hours or on high heat for 3-4 hours
Add salt/pepper to taste if needed
To serve: Sprinkle the soup with mozzarella cheese while its still warm and enjoy with homemade garlic parmesan cheese bread on the side.