Combine the butter, onion, and sausage in a Dutch oven and cook it over medium heat until the onion is tender and the sausage is slightly golden.
Drain off any excess grease if necessary.
Add in the potatoes, beef broth, cream, salt, pepper, and Italian seasoning.
Bring it to a boil and then reduce the heat, cover, and simmer for about 10-15 minutes, or until the potatoes are just fork tender.
Add in the spinach and cook for a couple more minutes.
In a small glass, whisk together the tapioca starch and the cold water until it is smooth. Add this slurry to the soup. Cook for a couple minutes while the soup thickens.