Cook the Italian sausage and red pepper flakes in a Dutch oven over medium-high heat until crumbly, browned, and no longer pink, 10 to 15 minutes. Save a couple of tablespoons of grease then Drain and set aside.
In reserved drippings, Stir in the onions and garlic; cook until onions are soft and translucent, about 5 minutes. Add Mushrooms cook until tender.
Pour the chicken broth into the Dutch oven with onion, mushroom mixture; bring to a boil over high heat. Add the potatoes, and boil until fork tender, about 20 minutes. Reduce the heat to medium and stir in the heavy cream and the cooked meats; heat through. Mix the spinach into the soup just before serving. Serve with polenta or Garlic Toast.