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).

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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

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.

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

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