Pour chicken broth into a large pot along with the chopped potatoes. Bring to a boil and then simmer for 15 minutes, or until tender.
Set a saute pan on the stovetop over medium heat and add Italian sausage and chopped onions. Crumble the sausage as it cooks and saute until the sausage is well browned and the onions are tender. Add minced garlic and cook an additional minute.
Reduce the heat to medium-low and sprinkle flour over the mixture. Continue to cook, while stirring, for two minutes. Then remove from heat.
Once the potatoes are tender, puree them until smooth with the chicken broth using an immersion blender or a countertop blender.
Add the crumbled sausage mixture to the soup pot along with heavy cream, fresh spinach, and black pepper. Taste for salt and adjust as needed. Bring the soup to a simmer, then serve.
Notes
This creamy soup can be kept warm in the slow cooker for extended periods of time, if needed.