Melt the butter in a large saucepan over medium heat, add the onion, leeks, and potatoes, and saute gently for 4-5 minutes. Add the vegetable stock and bring to a boil. Reduce heat to medium and simmer, covered, for 15 minutes.
Using a potato masher, mash until potatoes just begin to break down and soup is thickened. Season with salt and pepper. Swirl with the cream if using and garnish with the chives. Serve.