What You’ll Need: 3 sirloin steaks 1 white onion, diced 1 green pepper, diced 1 large portobello mushroom, diced 2 cloves garlic, finely chopped 1 tsp salt 1 tsp pepper 9 slider buns, cut in half (and held together if possible) 1/8 cup unsalted butter, melted 6 slices jarlsberg cheese 18 slices provolone cheese 1/4…