Prepare the Bacon: Heat a 5½-quart Dutch oven or large heavy pot over medium-high heat. Add the chopped bacon and cook, stirring occasionally, for about 5 to 7 minutes, until crispy. Use a slotted spoon to transfer the bacon to a plate lined with paper towels.
Brown the Sausage: Carefully pour out most of the bacon fat, leaving about 2 tablespoons in the pot. Return the pot to medium-high heat and add the Italian sausage. Break it apart with a wooden spoon as it cooks, stirring frequently for 6 to 8 minutes, until fully browned. Transfer the sausage to the plate with the bacon.
Cook the Onions and Garlic: Lower the heat to medium and add the diced onion to the pot. Cook, stirring occasionally, for about 5 minutes, or until the onion becomes soft and golden. If the pot seems dry, drizzle in a little olive oil. Stir in the minced garlic and cook for another minute, until fragrant.
Thicken the Base (Optional): If using flour for a thicker soup, sprinkle it over the onion and garlic mixture. Stir constantly for about 1 minute to incorporate the flour and prevent lumps.
Add the Liquid: Pour in the chicken broth and water, stirring to deglaze the pot by scraping up any browned bits from the bottom. Bring the mixture to a boil.
Cook the Potatoes: Add the sliced potatoes to the boiling broth. Reduce the heat slightly and let them simmer for 12 to 14 minutes, or until they are tender but not falling apart.
Incorporate the Kale and Sausage: Add the chopped kale to the pot, stirring to combine. Return the cooked sausage to the soup and bring everything to a gentle boil again.
Finish with Cream and Seasoning: Stir in the heavy cream and season with salt and pepper to taste. Adjust the seasoning if needed, and let the soup simmer for a few more minutes to meld the flavors.
Serve and Garnish: Remove the pot from the heat. Ladle the soup into bowls and top each serving with the reserved crispy bacon and freshly grated Parmesan cheese. Serve warm with crusty bread for dipping.
Storing Leftovers: Any leftover soup can be stored in an airtight container in the refrigerator for up to 4 days. Reheat gently on the stovetop or in the microwave before serving.