In a large pot, melt the butter over medium-high heat.
Add the onion, carrot, celery, and salt. Cook, stirring from time to time, until softened, about five minutes.
Stir in garlic, herbes de Provence, black pepper, and red pepper flakes, and cook for one minute.
Add flour and stir to coat well. Cook one minute more.
Stir in tomato paste.
Pour in broth and crushed tomatoes and stir well, scraping up any bits from the bottom of the pot and allowing the flour-coated vegetables to mingle with the liquid.
Nestle the parmesan rind into the broth.
Bring to a boil, then reduce heat and simmer for five minutes.
Stir in tortellini and simmer according to the length of time on the package. (The brand I typically use cooks for seven minutes.)
Stir in spinach and cooked chicken and simmer until spinach is wilted and chicken is warmed through.
Serve with grated parmesan to pass at the table if you like.