Cut beef with the grain into 1 1/2 inch-wide strips.
Cut each strip across the grain in 1/8 inch-thick slanting slices.
In a bowl, combine cornstarch, soy, sherry, water, andsalt.
Add beef and stir to coat, then stir in 11/2 teaspoons of the oil and let stand for 15 minutes to marinate.
Prepare cooking sauce and set aside.
Heat a wok or wide frying pan over high heat.
When pan is hot, add 2 tablespoons of the oil.
When oil begins to heat, add ginger and garlic and stir once.
Add beef and stir-fry until meat is browned on the outside but still pink within (about 11/2 minutes); remove from pan.
Heat the remaining 1 1/2 tablespoons oil.
Add celery and onion and stir-fry for 1 minute.
Add green pepper and stir-fry for 1 minute, adding a few drops water if pan appears dry.
Add tomatoes and stir-fry fori minute.
Return meat to pan.
Stir cooking sauce, add to pan, and cook, stirring, until sauce bubbles and thickens.