Brown the sausage. In a skillet over medium heat, cook the Italian sausage, breaking it into crumbles, until no longer pink.
If the sausage releases a lot of fat, spoon off excess, leaving about a tablespoon for flavor. Transfer sausage to the slow cooker.
Sauté aromatics (optional but recommended). In the same skillet, add onion and a drizzle of olive oil if needed. Sauté 3–4 minutes until translucent.
Add garlic and cook 30 seconds more. Transfer to the slow cooker.
Load the veggies. Add bell pepper, cauliflower, and zucchini to the slow cooker. Pour in the diced tomatoes with their juices and the chicken broth.
Season it up. Stir in oregano, basil, red pepper flakes, and fennel seeds.
Add a pinch of salt and several grinds of black pepper.
Slow cook. Cover and cook on Low for 6–7 hours or on High for 3–4 hours, until the vegetables are tender and flavors have melded.
Finish with greens and cream. Stir in spinach (or kale) and cook 5–10 minutes until wilted. Turn off heat and stir in heavy cream. Taste and adjust salt and pepper.
Serve. Ladle into bowls.
Top with Parmesan and fresh herbs if you like. Enjoy hot.