Potatoes act as their own thickeners. If you want this soup extra thick cut up an extra potato to cook with the soup. You can either leave the soup with the extra potatoes or before you add the milk, spoon out roughly a cup of potatoes and mash them up with a potato masher or blend them with an immersion blender and stir them back into the soup. You could also ladle out a 1/2 cup of soup liquid before adding the milk and whisk in 2-3 Tbs. of potato starch. Whisk that mixture into the soup.