Heat grill and place grill-top griddle on rack. Heat oil and butter in griddle over high heat. Add mushrooms and cook until the mushrooms are golden brown. Season with garlic salt and pepper to taste.
Heat oil in medium sauté pan over high heat. Add peppers and cook until soft. Season with salt and pepper.
Heat butter and oil in large sauté pan over medium heat. Add onions and cook slowly until golden brown and caramelized, stir occasionally. Combine onions, peppers and mushrooms into one pan and stir until well mixed.
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.
Slice four, one-inch-thick vertical slices of King's Hawaiian Bread from the round loaf and lightly brush both sides of each with olive oil. In a separate pan or grill, toast each side at medium heat until golden brown (1-2 minutes).
When all of the meat is cooked, cover each bread slice with provolone cheese. Combine the mushrooms/onions/peppers mix 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.