Heat a large Dutch oven over medium-high heat. Add the Italian sausage and diced yellow onion, breaking up the meat with a wooden spoon as it cooks for 6–7 minutes until deeply browned. Drain any excess fat, leaving about 1 tablespoon in the pot.

Add the minced garlic, Italian seasoning, red pepper flakes, tomato paste, and flour. Stir constantly for 90 seconds until the tomato paste darkens into a rich brick red.

Slowly pour in the chicken broth while scraping up the golden brown bits from the bottom of the pot, then stir in the undrained fire-roasted diced tomatoes. Bring to a boil and simmer for 5 minutes.

Add the refrigerated cheese tortellini to the simmering broth and cook for 4–5 minutes until plump and tender.

Reduce heat to low and stir in the heavy whipping cream, fresh baby spinach, and freshly grated Parmesan cheese. Simmer for 2 minutes until the spinach wilts and the broth turns velvety smooth. Ladle into warm bowls and top with extra Parmesan.