Heat the oil on a griddle over medium-high heat and cook the onion with 1/4 teaspoon salt for 7 minutes until browned.
Add the shaved beef, remaining salt and black pepper and cook for 3 to 4 minutes until browned and at least 145°F.
Chop the filling into short ribbons and cool it for 3 minutes.
Split the connected rolls, layer the bottoms with provolone and steak mixture, and replace the top slab.
Brush with butter and bake at 350°F for 12 to 15 minutes until the cheese melts.
Rest for 5 minutes and cut into 12 sliders.