Preheat the oven to 380°F with a rack in the center position. Heat the oil in a large cast iron pan over high heat. Once the oil is glistening, working in batches, add the beef in a single layer and season with ½ teaspoon of salt. Cook, undisturbed, until meat begins to brown, about 2 minutes. Stir and continue cooking until the beef is cooked through, about 2 minutes more.
Using a slotted spoon, transfer the beef to a plate. Reduce the heat to medium. Add the onions to the same pan and season with remaining ½ teaspoon of salt. Cook, stirring often, until browned and softened, 5-8 minutes. Meanwhile, in a small bowl, combine mayonnaise and garlic.
Spread 1 tablespoon of butter on the cut sides of each hoagie roll. Arrange rolls, cut sides up, on a sheet pan and bake until toasted, about 4 minutes. Remove from the sheet pan from the oven and spread 1 tablespoon of garlic mayo on the cut sides of each roll. Return beef to the pan with the onions and season with black pepper. Cook, stirring, over medium heat until warmed.
Remove the pan from heat and top the beef mixture with provolone in a single layer. Once the cheese begins to melt, divide the cheese and meat between toasted hoagie rolls. Serve immediately.
Note: You’ll want to slice the beef as thin as possible. The best way to do this is by freezing it for 30 minutes to an hour, then slicing with a sharp knife.