Brown the sausage: Heat the olive oil in a 5- to 6-quart Dutch oven over medium-high heat. Add the Italian sausage and cook for 6 to 8 minutes, breaking it into small crumbles. Continue until browned, no pink remains, and pork sausage reaches 160°F (71°C).

Drain excess fat if needed: If more than about 2 tablespoons of rendered fat remain in the pot, carefully spoon off the excess. Leave a light coating in the pot to flavor the vegetables.

Soften the vegetables: Add the onion, carrots, and celery. Reduce the heat to medium and cook for 5 to 7 minutes, stirring occasionally, until the onion is translucent and the carrots begin to soften.

Add the garlic and seasonings: Stir in the garlic, Italian seasoning, and optional red pepper flakes. Cook for about 30 seconds, until fragrant.

Cook the tomato paste: Add the tomato paste and stir it into the sausage and vegetables. Cook for 1 to 2 minutes, until it darkens slightly and coats the mixture.

Add the broth: Pour in the chicken broth, scraping the bottom of the pot to release any browned bits. Add the salt and black pepper, then increase the heat to medium-high.

Bring to a simmer: Once the broth reaches a gentle boil, reduce the heat to medium-low and simmer for 5 minutes. The carrots and celery should be partially tender but not completely soft.

Cook the ditalini: Stir in the dry ditalini: Simmer uncovered according to the package timing, usually 8 to 10 minutes, stirring every couple of minutes so the pasta does not settle on the bottom.

Check the pasta: The ditalini should be just tender with a slight bite in the center. If the soup becomes unusually thick before the pasta is ready, add 1/2 cup (120 ml) additional hot water or broth as needed.

Add the cream: Reduce the heat to low and stir in the heavy cream. Heat for 2 to 3 minutes until steaming, but do not let the soup boil vigorously.

Melt in the Parmesan: Add the 3/4 cup (68 g) Parmesan in two or three additions, stirring after each one until melted. The broth should look creamy and cohesive without clumps of cheese.

Wilt the spinach: Stir in the chopped spinach and cook for 1 to 2 minutes, until the leaves are wilted and bright green.

Adjust the consistency and seasoning: Taste the soup: Add more salt or black pepper only if needed. If it has thickened more than desired, stir in a small amount of warm broth or water until it reaches a creamy but spoonable consistency.

Rest briefly: Remove the pot from the heat and let the soup stand for 5 minutes. It will thicken slightly as the pasta absorbs more broth.

Garnish and serve: Ladle the soup into six bowls. Top with additional Parmesan, parsley, and crushed red pepper flakes as desired, and serve warm.