Cut steak into thin strips.
In a large skillet (oven-proof if desired) brown meat in oil, using high heat.
Add onions. Saute until tender.
Blend soup, sherry, liquid from mushrooms, and garlic salt.
Pour over steak.
Add mushrooms.
Reduce heat; cover and simmer for 1 hour or until steak is tender (or cover and bake at 350°).
Serve over beds of rice.