Using the sauté function on your slow cooker, or a large skillet on the stovetop over medium-high heat, cook the sausage until browned, breaking it into pieces as it cooks.
Add the onion, carrots, and celery and cook until softened, about 5 minutes. Add the garlic, Italian seasoning, and salt and cook 1-2 more minutes.
If you browned everything on the stovetop, transfer it to your slow cooker.
Stir in the northern beans and chicken broth. Cover and cook on LOW for 6-8 hours or HIGH for 3-4 hours.
In the lasts 10 minutes of cooking, stir in the chopped spinach, heavy cream, and Parmesan. Cook until the spinach is wilted and the soup is heated through.
Serve warm, topped with grated Parmesan cheese. Enjoy!