Heat oil or butter in a pan and saute mushrooms for 2-3 minutes. Transfer mushrooms to another pan and keep warm over low heat.
Heat a little more butter and oil and put in strips or beef in skillet and cook fast, turning or stirring 3 or 4 min, or until just rare, not browned. Put meat with mushroom, sprinkle lightly with garlic salt.
Cook celery and onion in butter until transparent.
Add green pepper and cook 2 more min.
Vegetables should still be crisp. Put all together in the larger skillet or pan; drizzle 2 or 3 tbsp soy sauce over all, add salt and pepper and serve at once.
