I added some cream to heated soup, which spread and floated to the top:
Leek and Potato Soup
adapted from Mastering the Art of French Cooking by Julia Child
3 to 4 cups or 1 lb. peeled, sliced or diced potatoes
3 cups or 1 lb. thinly sliced leeks, including the tender greens
1 quart water
1 quart vegetable broth
2 teaspoons salt
6 tablespoons heavy cream
freshly ground black pepper
Simmer the potato and leek in the water and broth, partially covered, for 40 minutes or until the vegetables are tender.
Mash the vegetables in the soup with a fork, pass through a food mill, or use a blender or food processor to puree. Season to taste with salt and pepper. Set aside uncovered until just before serving, then reheat to a simmer.
Off heat, before serving, stir in the cream. Garnish with parsley or chives, if desired.