Heat a saucepan over medium heat, then add the Italian Sausage and 1/2 cup water. Use a spatula to break into small chunks, then cook over medium until no longer pink, or about 5 minutes. Drain the water from the sausage and set aside.
In the same saucepan, heat the olive oil over medium heat and add the diced onion. Saute until caramelized, or about 5 minutes, then add the minced garlic and continue to cook for another 1-2 minutes until transparent.
In a large dutch oven, add the cooked sausage, onions and garlic, and the chicken broth. Bring to a boil.
Reduce to a simmer, then add in the diced carrots and potatoes and cook until tender, about 20 minutes. Once the potatoes are tender, add in the salt, pepper, spinach and milk. Bring to boil again.
Meanwhile, in a small bowl combine the cornstarch with the cold water and stir together until smooth. Add to the soup slowly, stirring until the soup thickens.
Once the soup is thickened and warmed to your liking, remove from the heat and ladle into bowls. Serve with grated Parmesan cheese if desired. Enjoy!