In a large skillet over medium heat, cook your thinly sliced onion, red bell pepper (if using) and mushrooms until they are tender. This usually takes me between 5 and 10 minutes depending on how soft/tender I want my veggies.
Move the cooked veggies to a plate to be used later.
If you were able to get thinly sliced ribeye from your butcher you can skip the next step because you're ready to cook.
If you have a whole unsliced ribeye, you will need to freeze it for an hour or two so that you can slice it thinly. After your steak has been in the freezer for 1 to 3 hours, remove it and get a very sharp knife and slice very thin pieces.
When all your meat is sliced you can start cooking.
In a large skillet or preferably cast iron griddle over medium high heat, add a little vegetable or olive oil to your pan. When the oil is shimmering add the steak and group it into "sandwiches." So if you're making two sandwiches, group the meat into two piles. Salt and pepper your piles of meat.
Leave the meat where it is for 4 minutes. After that time has elapsed, start moving it around, flipping and even chopping the larger pieces with your spatula. The real Philly cheesesteak joints really chop at the meat, to get it into small bite size pieces. Be careful and do not ruin your pan with a metal spatula if your pan has a non-stick coating.
Continue flipping and moving the steak around until all of the meat seems to be cooked.
Add your veggies to your steak. Divide the cooked veggies evenly between your meat piles.
With your spatula or tongs incorporate the meat and veggies until they are combined and mixed through.
Slice your sub/hoagie rolls 2/3rds of the way through and spread the pimento cheese on the inside of each one.
line the meat and veggies on the skillet/griddle up so that they are about the length of each of your buns. Open each bun and lay it on top of the still cooking meat/veggies.
Using a spatula, get under the meat and with your other hand grab the bun and flip the sandwich out of the pan, attempting to keep all of the meat and veggies in the sandwich. Do this for your other sandwiches as well.
Enjoy your pimento cheesesteak!
You can use this recipe with Cheez Whiz or any other cheese spread as well, just spread the cheese spread inside the bun as the meat is almost cooked through.
If you want to use sliced cheese, just lay the slices of cheese on top of the meat when it's cooked through and give it another minute or two for the cheese to start melting.