Melt the butter in a large saucepan over medium heat, add the onion, leeks and potatoes, and saute' gently for 2-3 minutes, until softened but not browned. Pour in the stock, bring to a boil, then reduce the heat and simmer, covered, for 15 minutes.

Transfer the mixture to a food processor or blender and process until smooth. Return to the rinsed-out saucepan.

Reheat the soup, season to taste with salt and pepper, and serve in warmed bowls, swirled with the cream, if using, and garnished with chives.