You want to begin by slicing your steak lengthwise against the grain. Then slice those long strips in half so you have smaller strips. You don't want the strips to be too thick or too thin. Place all the steak strips in a large bowl so we can begin to marinate the beef.

Combine the beef with unsalted butter cubes, olive oil, salt, and black pepper. Gently mix everything together until combined. Cover and place in the fridge for a couple of hours but ideally best if you do it overnight.

On the day you are ready to cook the steak, you can prep your toppings. Once all the toppings are ready, set aside. Take the marinated steak out of the fridge so it can be at room temperature for at least half an hour.

Prep your sub bread by slicing into it so you have a hinge created. Lay flat side down in your panini press or grill and begin to toast for a few minutes.

On a flat top griddle, cook up the marinated steak with the green peppers, mushrooms, and onions.

Once the beef and toppings are nearly finished cooking, you can line them up into a straight line on your griddle or grill. Next, pour any fat that has dripped out back on top of the steak and season again with salt and pepper. Then, place your cheese over the top and cover with a heat-proof aluminum pan or a large pot lid to steam the cheese.

While your cheese is melting, you can prep your sandwich. You should have your toasted sub roll at this point ready to go. I spread a bit of mayonnaise on both sides. Then, I top it off with some shredded iceberg lettuce and a few banana peppers, and roasted red peppers.

Use a bench scraper or large plastic scraper to move the steak and toppings from your griddle to your sub. There is no clean or easy way to do this. It will be messy, but try your best to get it all in your sandwich.

Your sub is ready to go at this point, but to make it easier to eat I like to roll it up tightly in deli paper or aluminum foil and slice it in half diagonally. Enjoy!