In a heavy skillet, melt 1 tablespoon of butter over medium heat.
Add the onion, green pepper, mushrooms and garlic.
Season with salt and ground pepper.
Cook the vegetables, stirring occasionally for about 15 minutes or until tender.
Meanwhile, cut the steak into paper thin slices.
In a separate heavy skillet, melt the remaining butter and cook steak over medium-high heat. Season with salt and a bit of pepper, stirring occasionally, for 3-5 minutes until browned.
Slice each sub or hoagie roll in half lengthwise, being careful not to slice completely through the roll. Line each roll with 2 slices of provolone cheese.