Soak lap cheong in a pan of hot water for about 10 minutes. Casings should puff up. Remove casings and slice at a diagonal.
Wash and drain Chinese leeks. Cut off the tops and remove any bruised outer dark green leaves. Then slice each leek diagonal.
Heat a wok or large pan. Fry lap cheong until lightly brown, about 2 minutes. There is no need to use any oil. Remove fried lap cheong, leaving the oil in the wok.
Add garlic and sauté for about 30 seconds. Then, add leeks, soy sauce, salt, and ground pepper. Stir fry for about 3 to 4 minutes.
Add 2 tablespoons of water if it appears too dry.
Return lap cheong to the pan. Stir to get all ingredients well mixed.
Add cornstarch mixture and continue to stir fry. Turn off heat. Drizzle sesame oil over vegetables.
Remove and serve immediately.