Melt butter in large sauce pan. Saute onions in butter until the begin to turn translucent. Add leeks, cover pan and sweat for about 5 min. Add potatoes and broth. Bring to a boil, then lower heat to simmer for 10 min. Remove veg and puree in blender. (This step can also be CAREFULLY done with immersion blender). Return pureed veg to stock, add salt and pepper. Chill. When ready to serve, add cream and Worcestershire.