In a saucepan bring the broth to a boil with the onion. Boil the mixture for 5 minutes, and let it cool.
In a food processor chop fine the garlic and the chilies; add the cream cheese, sour cream and cumin. Blend the mixture until it is combined well. With the motor running add the broth mixture in a steady stream. Blend the mixture until it is combined well, and transfer it to a large bowl. Stir in the half-and-half, white pepper and salt to taste, and chill the soup, covered, for at least 2 hours or overnight.