01 - Heat oil in a large skillet over medium-high heat.
02 - Add the sliced onions (and bell peppers, if using) and sauté for 5–7 minutes until softened and lightly caramelized.
03 - Push the vegetables to one side of the skillet.
04 - Add the thinly sliced beef to the empty side and cook for 2–3 minutes, breaking it apart with a spatula, until just browned.
05 - Season with salt and black pepper to taste. Mix the beef and vegetables together.
06 - Divide the beef and vegetable mixture into four piles in the skillet. Top each pile with two slices of provolone cheese.
07 - Let the cheese melt for about 1 minute.
08 - Slice the hoagie rolls open and place them over each pile to steam slightly.
09 - Carefully scoop the filling into the rolls using a spatula.
10 - Serve hot, with extra cheese sauce if desired.