Heat a Dutch oven over medium high, add oil if needed, then cook sausage until well browned with crisp bits forming on the bottom; break into small pieces for even bites. If excess fat pools more than 2 tablespoons, spoon it off. 6 to 8 minutes.
Add onion, carrots, and celery with a pinch of salt and cook until edges turn translucent and fond loosens; stir in garlic, Italian seasoning, and red pepper flakes for 30 seconds until fragrant. 5 minutes.
Stir in tomato paste and cook until brick red and sticky for concentrated flavor. 1 minute.
Pour in stock and add bay leaf; bring to a gentle boil, then reduce to a lively simmer until vegetables are tender and flavors meld. 10 minutes.
Add gnocchi and simmer until they float and feel springy with a tender center when pressed between spoon and pot side. 2 to 4 minutes.
Reduce heat to low. Whisk cornstarch into cream if using, then ladle a cup of hot broth into the cream to temper, whisking smooth; stir back into the pot and add spinach until just wilted. Do not boil after cream goes in. 2 minutes.
Remove bay leaf, taste, and adjust salt and pepper; ladle into warm bowls and top with grated Parmesan and a few black pepper grinds.