Cut steak diagonally across grain into thin strips. Place into a large non-metal bowl. Cover with onion, garlic, ginger, and sugar. Combine soy sauce and sherry. Pour over meat mixture. Let stand at room temperature for 2 to 3 hours.
Remove beef strips and set aside marinade and vegetables.
Heat oil in work over high heat. Stir-fry beef strips a few at a time until lightly browned. Place on a hot platter.
Add marinade and vegetables to wok and bring to a boil. Simmer for 10 minutes. Remove vegetables and other bits and reserve as a side dish.
Add cornstarch to water. Add to marinade and stir until liquid thickens.
Pour marinade over steak strips and serve over rice.