Heat butter and oil in large sautée pan over medium heat.
Add onions. Once onions are translucent, reduce heat to medium-low and cook slowly until golden brown and caramelized, about 45 minutes; stir occasionally.
While onions are cooking, heat oil and butter in another large sautée pan set over high heat. Add mushrooms and cook until the mushrooms are golden brown and liquid has evaporated.
Season with garlic salt and pepper to taste. Then, remove from pan and set aside.
In same large sautée pan, heat oil over high heat.
Add peppers and cook until soft.
Season with salt and pepper to taste.
Combine peppers, mushrooms and onions into one pan and stire until well mixed. Keep other pan to cook steak.
Heat a heavy griddle pan over medium heat. Wipe griddle with a drizzle of oil using a paper towel.
Cook thinly-sliced steaks until brown but not crisp, about 2 minutes on each side. While browning, sprinkle them with garlic salt and pepper. Cook steaks in single layers in 2 batches and tenderize by cutting into meat with the side of your spatula while they cook.
Carefully remove the entire 12-pack of rolls from the packaging without separating rolls.
Make one horizontal slice through the entire dozen, separating the tops from the bottoms. Brush the insides with olive oil. Under the broiler, toast both sides until golden brown, about 1-2 minutes.
When all of the meat is cooked, cover each bread slice with provolone cheese.
Combine the mushrooms/onions/peppers mixture with the meat in the griddle and mix together with your spatula. Pile the filling on top of the cheese on each bread slice. The heat from the meat and onions will melt the cheese.