Heat the olive oil in a Dutch oven or large soup pot over medium heat until shimmering. Add the onion, and sauté until softened, 3-4 minutes. Add the sausage and cook until mostly brown, 5-7 minutes, breaking it up as you go.
Add the potatoes, chicken broth, dried Italian seasoning, and red pepper flakes. Raise the heat and bring to a boil. Once the boiling point is reached, reduce the heat to low and simmer, covered, until the potatoes are tender, 15 to 20 minutes, adding the chopped greens in the final 3 minutes or so. So as not to overcook, check the potatoes a minute or two early and then every few minutes until done. They should be tender but not mushy.
Ladle into bowls and serve with a sprinkle of Parmesan cheese, if desired.
Notes
*You may substitute plain ground turkey or use leftover Thanksgiving turkey. In the latter case, plan on about 3 cups of shredded or chopped meat and add with the potatoes. **I’ve used sweet potatoes, Yukon gold potatoes (not Russet, which won't hold their shape as well), and a mix for this soup, so feel free to use what you enjoy. I like to keep the skin on the potatoes, but you may peel if preferred. I’ve also used up to a quarter pound more or less of the potatoes with good results. Feel free to take a little leeway in either direction. Potatoes (especially sweet potatoes) can grow quite large, so weighing at the store, when in doubt, can be helpful. ***Homemade chicken stock may be used in place of store-bought. In that case, I add between 1¼ - 1½ teaspoons kosher salt at the end. If using table salt, which has finer granules and is therefore saltier, plan on ¼ teaspoon less. ****Chopping the baby greens will prevent the cooked greens from seeming stringy. Also, a half bunch of kale provides another alternative to the baby kale or spinach. If using, strip away the tough stems and chop the remaining leaves into bite-size pieces. I recently used a 5-ounce package of mixed baby kale and spinach, which worked very well.A few more things...
•A 15-ounce can of beans (rinsed and drained) could be added. The outcome will be less brothy, so you may wish to have an extra cup or two of broth on hand. Conversely, you could reduce the sausage and potatoes by a ¼ pound each.Storage: Refrigerate leftovers in an airtight container for up to 4 days.