Bring 1/2 cup of broth to a boil in a large pot. Add onion and leek, simmer until very soft and most of liquid has evaporated.
Add broccoli and potatoes. Simmer until very soft, about 30-40 minutes. Remove some larger pieces of brocolli for texture if preferred.
Add sour cream and milk. Puree with immersion blender. Return larger peices of broccoli. Serve hot or the soup can be chilled to serve in summer. (I adopted this recipe from a chilled soup version, but have not tried it cold.)
Two notes: I could not get the analyzer to log the broth as vegetable, but that will reduce the calories by about 20. And, the soup is also quite tasty when about 1/4 cup of nutritional yeast is stirred in with the milk and sour cream. It enhances a cheezy-type flavor (not unnatural tasting, it just wouldn't fool a cheese lover) and packs in the B vitamins. It will add about 20 calories per serving.