In a large stockpot add broth and corn and bring to a simmer. After 5 minutes of simmer add water chestnut, salt, pepper and sugar to taste. Simmer for 5 minutes longer. Thicken with roux and simmer to 2 minutes. Add in the beaten egg and whisk around with the heat turn off. Blend in the crab meat and top with minced green onion before serving.