Begin by cutting the end off the leek and then cut the leek in half lengthwise. Chop the leek into small pieces; this makes the leek easier to wash as they are notoriously dirty. Wash the leeks very well in cool water and rinse several times.
Step 2
Once the leeks are washed, preheat a large saucepan over medium-high heat. Add in the butter and allow to melt, then swirl the pan to coat it. Add in the chopped leeks and onions. Cook for 3-4 minutes or until translucent.
Step 3
Add in the water and cauliflower florets. Cook for 10-12 minutes at a simmer until the cauliflower is tender.
Step 4
Place ½ of the mixture into a high-powered blender and puree until smooth. Pour the soup into another bowl and repeat with the remaining soup.
Step 5
Combine the soup and place it all back into the saucepan and add the cream and sour cream. Use a whisk if necessary to smooth the sour cream. Chill and serve when the soup is around room temperature.