In a large skillet add oil and saute mushrooms and onions. I like to start out with the heat on high to get a deep golden color, lower heat and cook until mushrooms and onions are tender. Add steak, mix just until combined. Set a side.
Butter the bread on one side, top it with about 2 1/2 tbsp of cheese, top the cheese with the onion and steak mixture, sprinkle with 2 1/2 tbsp of cheese. Top it with the second slice of buttered bread. Now butter the bread on the outside. Do this with all the sandwiches. When you're done all 4 sides of each sandwich will be buttered.
Carefully transfer the sandwiches to a pan buttered side down. Butter the bread. Cook on both sides until the cheese is 'melty and gooey.'