Add a small amount of oil to a preheated griddle and spread it evenly with a spatula. Sauté the diced onions until translucent, season with salt and pepper, and move them to a cooler part of the griddle to keep warm. Add the thinly sliced ribeye to the hot griddle in a single, even layer. Using a spatula and a bench scraper, chop the steak as it cooks by pulling and scraping pieces in a fluid motion. If cooking in a stovetop skillet, chop the meat into bite-sized pieces before adding to the pan. Cook the meat just until no longer pink, stirring often. Do not overcook. Season the steak with salt, pepper, or an all-purpose seasoning and mix in the sautéed onions. Divide the meat into 3 even piles on the griddle, each the length of a hoagie roll. Place 3 slices of cheese on top of each pile (if using Cheez Whiz, skip for now). Immediately place hoagie rolls, opened and face-down, over each meat and cheese pile to steam for 2–3 minutes. Use a spatula to lift the steak and roll together to form the sandwich. If using Cheez Whiz, steam the rolls over the meat, then transfer meat to rolls and drizzle with warmed Cheez Whiz. Let rest for a few minutes before serving.