Brown the sausage: Heat a large skillet over medium-high heat. Add the Italian sausage and cook for 6–8 minutes, breaking it into bite-sized pieces as it browns.
Cook the aromatics: Stir in the onion and cook for 3–4 minutes until softened. Add the garlic, Italian seasoning, and red pepper flakes (if using), and cook for 30 seconds until fragrant. Stir in the tomato paste and cook for 1 minute.
Build the sauce: Pour in the chicken broth and cream. Scrape up any browned bits from the bottom of the skillet. Stir in the cannellini beans and baby spinach, then bring the mixture to a gentle simmer. Cook for about 5-10 minutes, allowing the flavors to meld and the sauce to thicken slightly.
Finish and serve: Remove the skillet from the heat and stir in the Parmesan cheese until the sauce is smooth and creamy. Taste and season with salt and black pepper as needed. Garnish with chopped parsley or basil and extra grated Parmesan, if desired.