In a skillet, heat the oil or butter over medium heat. Add the onion slices and cook until they're soft and lightly caramelized, about 5-7 minutes.

Raise the heat to medium-high and introduce the thinly sliced ribeye to the skillet. Lightly season with salt and black pepper. Cook for a couple of minutes, stirring occasionally, until the steak is just done.

Return the caramelized onions to the skillet. If you're using bell peppers, add those now too. Mix everything together until well combined.

Lay the provolone cheese slices over the steak mixture. Cover the skillet briefly to let the cheese melt, about 2-3 minutes.

Gently stir to combine the melted cheese with the meat and veggies. Spoon the mixture into the split hoagie rolls.