Sauté onions and peppers in olive oil for 4-5 minutes.
Add Italian sausage; cook, breaking up, until no longer pink. Stir in potatoes, tomato paste, Italian seasoning, red pepper flakes, and salt + pepper. Let cook for 1 minute.
Pour in broth and heavy cream. Cover pan with lid and simmer for 15-20 minutes, until potatoes are fork-tender.
Turn off heat and stir in parmesan and baby spinach.
