1Slide the steak into the freezer for 15 to 20 minutes so it firms up. This is what lets you slice it paper-thin.
2While the steak chills, heat 1 tablespoon of the oil in a large skillet over medium heat. Add the onions with a pinch of the salt and cook, stirring now and then, for 12 to 15 minutes, until soft and golden brown. Scrape them into a bowl and wipe out the skillet.
3Slice the chilled steak against the grain as thin as you can, about 1/8 inch. Toss the slices with the remaining salt, the pepper, and the garlic powder.
4Heat the skillet over medium-high heat until very hot and add the remaining tablespoon of oil. Cook the steak in two batches in a single layer, leaving it alone for about 1 minute to get a brown crust, then stir and chop it up with two spatulas for another minute or so, just until the pink is gone. Don't crowd the pan or the beef will steam.
5Return all the steak and the onions to the skillet and stir in the Worcestershire. Gather the mixture into 4 loose mounds, one for each roll, and lay 2 slices of provolone over each. Cover the skillet for 1 to 2 minutes, until the cheese is melted and draping over the meat.
6Spread the softened butter over the cut sides of the rolls and toast them cut-side down in a second skillet or on the cleaned edge of the first over medium heat for about 1 minute, until golden.
7Slide a spatula under each cheesy mound and scoop it right into a toasted roll. Serve right away while the cheese is still stretchy.