Slice pepper and onion into thin, long strips. Drizzle a little olive oil, salt and pepper and toss. Heat a grill and place pepper and onion slices, grilling for a few minutes until wilted and charred. Remove from the grill. Slice ciabatta rolls in half, place a slice of cheese on both sides of the bun. On one side of the bun, place 4-5 strips of flank steak, peppers and onion, on each roll, then sprinkle with a little basil. Cover with the other slice of cheese and bun. Place on a panini press and grill for several minutes until the cheese melts. Enjoy!