Heat half the oil in a skillet; saute the onions 3 minutes.
Add half the sherry and soy sauce and the sugar.
Cook for 10 seconds.
Remove from pan.
Cut the steak in narrow slices.
Toss with the cornstarch and the remaining sherry and soy sauce.
Heat the remaining oil and brown the meat in it.
Return onions to pan and cook 2 minutes.