Preheat the oven to 350°F. Line a baking sheet with aluminum foil and spray with cooking spray.
Slice the rolls in half horizontally to separate the top and bottom, keeping them together as one slab. Do not break into individual rolls.
Place the bottom half on the baking sheet.
Heat 1 tablespoon olive oil in the skillet. Season the steak with salt and pepper and cook over medium heat until done, remove from skillet, and place on top of the bottom slab of rolls.
Take about 10 pieces of your sliced onion and set them aside. In the same skillet heat the remaining 1 tablespoon olive oil and cook green pepper and remaining onion together until tender.
Place the green pepper and onion on top of the steak.
Top everything with 8 slices of provolone cheese. Place the other half of the rolls on top.
Melt butter and concentrated beef stock together. Stir to combine. Brush mixture over top of rolls. Finely dice reserved onion slices and sprinkle over top of rolls.
Bake in the preheated oven for 10 minutes, or until cheese is melted and the tops have started to brown. If necessary, cover the rolls with aluminum foil to prevent them from browning too much.
Slice into individual sliders and serve warm.