Preheat oven to 350°F. Dice the green pepper and onion, reserving about 2 tablespoons onion for topping.
Heat olive oil in a large skillet over medium heat. Add green pepper and most of the onion and cook 5 to 6 minutes, until softened. Remove from pan.
Add thin-sliced sirloin to the same skillet. Cook until just browned through, then sprinkle with garlic powder and cut any large pieces into slider-sized bites.
Slice the connected sheet of rolls horizontally and place the bottom half in a 9x13 baking dish.
Spread mayonnaise lightly over both cut sides of the rolls.
Layer sautéed vegetables, steak, and provolone over the bottom rolls. Add the top layer of rolls.
Brush melted butter over the tops and scatter reserved onion over the rolls.
Cover tightly with foil and bake at 350°F for 10 minutes.
Remove foil and bake 10 more minutes, until tops are golden and cheese is melted.
Rest 2 minutes, cut along the seams, and serve warm.
Notes
Freeze steak for 20 to 30 minutes before slicing so you can cut it thinly. Do not skip the light mayo layer; it keeps the rolls from drying out without making them taste like mayonnaise. Use a snug 9x13 dish so the rolls hold together. If the tops brown too quickly, tent loosely with foil.
Keyword hawaiian roll sliders, philly cheesesteak sliders