Place the steak in the freezer uncovered for 30 minutes to help slice.
Preheat the LoCo griddle with 3 cooking zones: sear, 400°F, and low.
When the steak is ready slice thinly giving you a pile of thinly sliced beef.
Dice the onion and set to the side.
Split the sub rolls in half.
Add oil and butter to the hot side of the grill then add the diced onions. Cook the onions 5-6 minutes until translucent and slightly browned. Then move the onions to the cooler side of the griddle.
Add more oil and butter then the thinly sliced steak. Season the steak with salt and pepper. Continue cooking and stirring occasionally until the meat is well browned.
Using the spatula scoop the onions up and mix them in with the cooked steak. When mixed well, divide the pile of steak and onions into 4 equal portions moved to the cooler side of the griddle.
Top each pile with two slices of provolone cheese and close the lid of the griddle to help the cheese melt.
Schmear some mayonnaise on the split sub rollas and place them on griddle split side down for 3-5 minutes or until they are nicely toasted.
Place the toasted sub roll on top of a pile of steak, onion, and cheese. Using the spatula scoop the steak mixture in to the sub roll. Do this for each pile of meat.
Place the stuffed sub roll in to a sheet of aluminum foil and wrap tightly. Let the cheesesteaks sit like this in the foil for 5-10 minutes so the rolls get soft and steamy. Slice in the middle and enjoy!