Place a large skillet over medium high heat. Drizzle with vegetable oil.
Once hot, add half the thinly sliced onion and cook, stirring often, until softened and golden, about 5 minutes.
Add half of the thinly sliced beef in a single layer. Cook for about 2 minutes before seasoning with salt, pepper, and garlic powder. Continue to flip and cook until browned on all sides, using a spatula to cut the slices of meat into pieces.
Separate the meat and onion mixture into two separate sides about the length of the roll being used. Cover each with slices of the cheese. Cover the pan just long enough to melt the cheese, about 30 seconds.
Slide a spatula under the meat and transfer to the inside of an open roll. Fill a second roll.
Repeat with remaining onions and beef. Serve immediately.