In a large pot, melt butter over medium heat.
Add the chopped onion and leeks, sautéing until soft, about 5 minutes.
Stir in the diced potatoes and cook for another 5 minutes.
Pour in the broth and bring to a boil.
Reduce heat and simmer for 20 minutes, or until potatoes are tender.
Use an immersion blender to puree the soup until smooth.
Stir in the heavy cream and heat through, but do not boil.
Season with salt and pepper to taste.
Garnish with fresh herbs if desired.
Serve hot and enjoy!