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. Drain and set aside.

Cook the bacon in the same Dutch oven over medium heat until crisp, about 10 minutes. Drain, leaving a few tablespoons of drippings with the bacon in the bottom of the Dutch oven. Stir in the onions and garlic; cook until onions are soft and translucent, about 5 minutes.

Pour the chicken broth into the Dutch oven with the bacon and onion 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 sausage; heat through. Mix the spinach into the soup just before serving.