Combine broth, 1 tablespoon of fish sauce, sugar and salt in a 3½ litre soup pot. Bring to the boil reduce heat and simmer.
Meanwhile, heat oil in a skillet. Add shallots and garlic and stir-fry until aromatic. Add crab meat, the remaining 2 teaspoons of fish sauce and black pepper to taste.
Stir-fry over high heat for 1 minute. Set aside.
Bring the soup to a boil. Add cornflour mixture and stir gently until the soup thickens and is clear. While the soup is actively boiling, add egg and stir gently.
Continue to stir for about 1 minute. Add crab meat mixture and asparagus with its canning liquid. Cook gently until heated through.
