Brown the Italian sausage in a large skillet over medium heat, breaking it into crumbles as it cooks. Drain excess fat and transfer the sausage to the slow cooker.
Add the diced onion, minced garlic, and sliced fennel to the same skillet. Cook for 3 to 4 minutes until the onion softens, then scrape everything into the slow cooker.
Add the shredded cabbage, chicken broth, diced tomatoes, 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 until the cabbage and fennel are tender.
Taste and adjust salt or pepper if needed. Ladle into bowls and top each serving with grated Parmesan cheese.