Brown the Italian sausage in a large skillet over medium heat, breaking it into small pieces as it cooks. Drain excess fat and transfer the sausage to the slow cooker.
Add the diced onion, minced garlic, chicken broth, white beans, Italian seasoning, salt, and black pepper to the slow cooker. Stir to combine.
Cover and cook on low for 6 hours or on high for 3 hours.
Stir in the chopped kale during the last 30 minutes of cooking so it softens but stays bright green.
Taste and adjust salt if needed. Ladle into bowls and top with grated Parmesan cheese before serving.