Heat a large skillet over medium-high heat. Add oil and 1 tablespoon of butter. Toss in the onions and bell peppers if using. Cook until soft and slightly golden, about 8 to 10 minutes. Scoop them onto a plate.

In the same skillet, add the remaining tablespoon of butter. Add the thin sliced beef in a loose layer. Sprinkle with salt, pepper, and garlic powder. Cook quickly, stirring and flipping until just cooked through, 3 to 5 minutes.

Return the onions and peppers to the skillet, mix well, and taste for seasoning.

Slice the slider rolls in half as one sheet if possible. Place the bottom half in a baking dish. Layer half the cheese, then the beef mixture, followed by the remaining cheese. Place the top rolls on.

Brush the top with melted butter and a light sprinkle of garlic powder. Bake at 350°F (175°C) until the cheese is melted and tops are golden, about 12 to 15 minutes. For extra toasty sliders, broil for 30 to 60 seconds, watching closely.