In a large pot, melt butter with olive oil, over medium heat.
Add leeks, and cook until translucent, about 5 - 8 minutes.
Add potatoes and toss with the leeks. Salt and pepper to taste.
Add broth and simmer until potatoes are soft, about 30-40 minutes.
With an immersion blender (or a conventional blender) process soup until smooth.
Stir in lemon juice and, if desired, cream. Add salt and pepper to taste.