Remove the beef from the refrigerator 1 hour before cooking.
Heat a grill pan over high heat.
Lightly pound the meat with a smooth meat pounder until it is even thickness throughout. Toss the meat with 1 teaspoon olive oil and season with salt and pepper. Once the grill pan is hot, place the meat on the pan and cook on both sides until cooked through, 3 to 4 minutes per side. (If necessary, cut the chain in half or thirds to fit in the pan.) Remove the meat from the grill and wrap in aluminum foil. Allow to rest while the onions are cooking.
Add the remaining teaspoon of oil to the grill pan, along with the onion. Saute the onions until they are tender and beginning to brown, 7 to 10 minutes.
Slice the beef into small strips and divide evenly among the hoagie rolls. Pour any juice from the meat onto the hoagie as well. Top the meat with the cheese, followed by the onions. Wrap the sandwiches in aluminum foil for 10 minutes, then serve.