In a large pot or Dutch oven, brown the Italian sausage over medium heat. Drain off any excess grease.
Add the sliced fennel to the pot and cook until softened, about 5-7 minutes.
Pour in the chicken broth and crushed tomatoes. Bring to a boil, then reduce heat and simmer for 20 minutes.
Add the ditalini pasta and cook until tender, about 8-10 minutes.
Stir in the baby spinach and cook until wilted, about 2 minutes.
Season with red pepper flakes and salt and pepper to taste.
Serve hot, garnished with grated parmesan cheese.