In a large stockpot, heat the oil over medium heat and saute the leeks 5 minutes or until tender. Add garlic and saute for 1 additional minute.

Add chicken stock and potatoes to the pot and bring to a boil. Reduce heat and simmer for 10 minutes, or until potatoes are tender. Remove from  heat and use an immersion blender to puree the mixture until smooth and creamy. Alternatively, transfer the soup to a food processor or blender and puree, then return soup to pot.

Add the chicken sausage and spinach to the soup and simmer for a few minutes until the spinach is wilted and the sausage heated through.

In a small bowl, whisk the flour and salt into the milk until smooth. Gradually add the milk mixture to the soup. Bring to a boil; cook and stir for 2 additional minutes.

Reduce heat to low until ready to serve. Top with grated Parmesan cheese, if desired.