In a Dutch oven or stockpot, cook sausage and onions over medium-high heat until onions soften and sausage starts to brown, 5 to 7 min. Stir in potatoes, black pepper and your choice of spice blend. Cook, stirring, for 3 more min. Add chicken broth and bring to a boil. Reduce and simmer, covered, until potatoes are tender but not falling apart, about 15 min. Meanwhile, combine cornstarch and water in a small bowl to create a smooth slurry. Slowly pour slurry into the soup pot, stirring continuously, and cook until soup is slightly thickened. In the final few minutes of cooking, stir in half and half and Parmesan. Remove from heat and allow soup to cool for 5 min. before serving. Top each serving with scallions, if using.