Stir-Fried Beef with Leeks

Occasionally I am asked by surprised students of mine how it is that leeks exist in China, because, they say, leeks are French, are they not? Well, of course they are not only French; leeks are well known and well eaten, and in Canton they are particularly big and fat, and their taste is strong and distinctive. They go well with strong-flavored meats such as beef and lamb. But since lamb is rarely eaten in Canton, I have combined leeks with beef for this recipe.