Preheat the oven to 350°F.
Prep your veggies. Chop the onion and green bell pepper.
Sauté the veggies. In a large skillet over medium heat, cook the onion and pepper for 5-7 minutes until slightly softened. Transfer them to a plate.
Cook the steak. Add the shaved steak and steak seasoning to the skillet, stirring as it browns.
Combine everything. Toss the peppers and onions back into the skillet with the steak, cooking for an additional 3-5 minutes to blend the flavors.
Assemble the sliders. Top with the slider roll tops. Slice the slider rolls in half, keeping the top and bottom intact.
Place the bottom halves into a casserole dish. Layer half of the steak and veggie mixture over the rolls, followed by 4 slices of provolone.
Add the remaining steak mixture, then tuck in the broken pieces of the last 2 slices of cheese for full coverage.
Melt the butter and brush it generously over the tops of the rolls.
Pop the casserole dish into the oven and bake for 10-15 minutes, until the cheese is melted and the rolls are golden brown.