Make Garlic Butter: Combine the softened butter and pressed garlic in a small bowl. Mix well and evenly spread the garlic butter over the cut sides of the hoagie rolls.
Toast the Rolls: Heat a skillet or griddle over medium heat. Place the rolls buttered-side down in the pan and toast for about 1–2 minutes, or until they are golden and crisp. Remove the rolls from the skillet and set aside.
Cook the Onions: Add 1 tablespoon of oil to the skillet and heat over medium. Add the diced onions, stirring frequently, and cook for about 8–10 minutes until softened and caramelized to a golden brown color. Transfer the onions to a bowl and set aside.
Cook the Steak: Turn the heat up to high and add the remaining tablespoon of oil to the skillet. Spread the thinly sliced steak evenly in the pan and let it cook undisturbed for about 2 minutes, until it begins to brown. Flip the steak, season it with salt and pepper, and cook for another 2–3 minutes, or until it is fully cooked through. Stir in the caramelized onions to combine.
Prepare the Cheesesteak Mixture: Divide the steak and onion mixture into 4 portions while still in the skillet. Lay 2 slices of provolone cheese over each portion and allow the cheese to melt from the residual heat. Turn off the heat once melted.
Assemble the Sandwiches: Spread a thin layer of mayonnaise on the toasted side of each hoagie roll. Place the open roll directly over one portion of the steak and cheese mixture. Use a spatula to scoop and flip the mixture into the roll, closing it as you go. Repeat for the remaining rolls and cheesesteak portions.