Combine the broth, 1 tablespoon of the fish sauce, the sugar and salt in a 3 litre soup pot. Bring to a boil. Reduce the heat and simmer. Meanwhile, heat the oil in a skillet. Add the shallots and garlic and stir-fry until aromatic. Add the crab meat, the remaining 2 teaspoons fish sauce and black pepper to taste. Stir-fry over high heat for 1 minute. Set aside. Bring the soup to a boil. Add the cornstarch mixture and stir gently until the soup thickens and is clear. While the soup is actively boiling, add the egg and stir gently. Continue to stir for about 1 minute. Add the crab meat mixture and asparagus with its canning liquid; cook gently until heated through. Transfer the soup to a heated tureen. Sprinkle on the coriander, scallion and freshly ground black pepper.