1. Brown the Italian sausage in a large soup pot or Dutch oven over medium-high heat, breaking it into crumbles, until browned and cooked through, 6–8 minutes. Spoon off all but about 1 tablespoon of the drippings.

2. Add the butter and diced onion to the pot. Cook, stirring often, until the onion is soft and translucent, 4–5 minutes. Stir in the minced garlic and cook until fragrant, about 30 seconds.

3. Stir in the tomato paste, Italian seasoning, crushed red pepper flakes (if using), kosher salt, and black pepper. Cook, stirring constantly, for 1 minute to bloom the spices and lightly toast the tomato paste.

4. Pour in the chicken broth and water, scraping up any browned bits from the bottom of the pot. Add the diced potatoes. Bring the soup to a gentle boil, then reduce the heat, cover, and simmer until the potatoes are just fork-tender, 10–12 minutes.

5. Stir in the ditalini pasta. Simmer the soup uncovered, stirring occasionally so the pasta doesn’t stick, until the pasta is al dente, about 8–10 minutes.

6. Reduce the heat to low. Stir in the heavy cream and whole milk until combined, then add 1 cup of the grated Parmesan and 2 tablespoons of the chopped parsley. Stir gently until the cheese has melted into the broth and the soup looks creamy. Avoid boiling after the dairy is added.

7. Taste and adjust the seasoning with a bit more salt and pepper if needed. Ladle the soup into bowls and top with the remaining 1/2 cup Parmesan and remaining parsley. Serve warm.