Vichyssoise

2 1/4 cups chicken broth
3 leeks (white part only), sliced
2 potatoes, peeled and sliced
1/2 medium onion, sliced
1/2 teaspoon salt
1/2 cup half-and-half
1/2 cup sour cream
Chopped chives

Combine broth, leeks, potatoes, onion and salt in a saucepan and bring to a boil. Reduce heat and cook gently for 45 minutes. Mash the mixture and force through a sieve. Add half-and-half and sour cream, stirring until smooth. Add more salt, if necessary. Heat to just below boiling and remove from heat. Cool, then chill. Serve cold, garnished with chives.