Heat a large pot over medium-high heat and spray with oil. Remove sausage from casing and crumble into pot and cook until browned, about 7 minutes, stirring often. Remove the sausage and set aside in a bowl, but leave the rendered fat in the pot
Reduce heat to medium. Add 1 teaspoon oil, and then the onion and garlic. Cook until the vegetables soften, about 5 minutes. Add the Italian seasoning and stir well to coat the vegetables.
Add potatoes and pour in the broth. Increase the heat to medium-high and cook until the broth just reaches a simmer, but before it begins to boil. Add the cooked sausage. Reduce the heat to medium-low, partially cover the pot with a lid, and simmer until the potatoes are cooked through; about 15 minutes.
Uncover the soup and stir in the spinach, and then taste the soup for seasoning. Adjust with more salt and pepper, as needed.
Reduce the heat to low, cover with the lid and cook about 5 minutes more.