In a large skillet over medium heat, add the Italian sausage, breaking it apart with a spatula. Cook for about 5–7 minutes until browned and fully cooked. Transfer the sausage to a 6-quart slow cooker.

Using the same skillet, add a splash of oil and sauté the chopped onion for 3–4 minutes until translucent. Stir in the minced garlic and cook for an additional minute. Transfer to the slow cooker.

Pour in the broth along with the cauliflower and kale. Sprinkle in the crushed red pepper flakes, salt, and pepper, and stir to combine.

Cover the slow cooker and set it to cook on high for 4 hours or low for 8 hours. Keep an eye on it for the last half-hour, ensuring the kale remains vibrant.

Once cooking time is complete, stir in the heavy cream and allow the soup to warm through for an additional 10 minutes before serving.