In a large soup pot over medium heat, cook the Italian sausage until browned and fully cooked. Break it into crumbles as it cooks, about 6–8 minutes. Transfer to a plate lined with paper towels to drain excess grease. If there’s too much fat left in the pot, remove some, leaving about 1 tablespoon.
In the same pot, add a splash of olive oil if needed. Sauté the diced onion for about 5 minutes until soft and translucent. Add garlic and cook for another minute until fragrant — don’t let it burn. This step builds the aromatic base for your soup.
Stir in the diced potatoes, then pour in the chicken broth. Scrape the bottom of the pot to release any browned bits for extra flavor. Bring to a boil, then reduce heat and simmer for 15–20 minutes, or until potatoes are fork-tender.
Return the cooked sausage to the pot. Stir in crushed red pepper flakes, salt, and black pepper to taste. Simmer for 5 more minutes to allow flavors to blend.
Pour in the heavy cream and stir gently. Add the chopped kale and let it wilt into the soup — about 2–3 minutes. Taste and adjust seasonings as needed.
Ladle the soup into bowls and top with freshly grated Parmesan or crumbled bacon if desired. Serve hot with crusty bread or garlic knots on the side.