Slash fat edges of steak. Slice pockets in each side, cutting almost to bone.
In a frying pan, cook chopped onion and garlic in 1 tablespoon of butter.
Add celery salt and pepper. Stuff pockets with onion mixture.
Mix wine and soy sauce; brush on steak.
Grill with hood closed 15 minutes, basting often.
Turn steak; grill 10 to 15 minutes longer. If you prefer your meat a bit more done add a few minutes cooking time to each side. Baste often.
Cook mushrooms and green onions in 2 tablespoons of butter until tender; spoon over steaks.