Heat olive oil in a large pot over medium heat.
Add chopped onions and sauté until translucent.
Stir in the leeks and cook for about 5 minutes.
Add the diced potatoes to the pot.
Pour in the broth and bring the mixture to a boil.
Reduce heat and let it simmer for 20 minutes or until potatoes are tender.
Use an immersion blender to puree the soup until smooth.
If desired, stir in heavy cream for added richness.
Season with salt and pepper to taste.
Serve hot, garnished with chopped chives.