We just tried this recipe (I found it in one of those Pillsbury magazine's that you pick up in the grocery store checkout aisle), and it is tasty. And, since flank steak doesn't seem to come in single pounds, and our hoagie breads are the closest thing that I could get to a demi French baguette and they only come in 8-packs, this recipe went a lot further. In fact, I just kept cutting off the other half of the flank steak for a week!
In 10" non-stick skillet, heat 2 tbsp of the oil over medium heat. Add onions, cook, stirring occasionally, 10 - 12 minutes or until deep golden brown. Add vinegar, 1/2 tsp of the salt and 1/4 tsp of the pepper; cook 1 minute longer or until liquid has evaporated. Remove from heat; set aside.
In small bowl, mix cheese, mayonnaise and 1/4 tsp pepper. Set aside.
Preheat broiler for a few minutes. Sprinkle steak with remaining 1/2 tsp of the salt and remaining 1/2 tsp of the pepper. Place steak on a broiler pan and broil a couple inches under the broiler for 8 - 10 minutes, or to desired doneness turning once. (Can be grilled instead). Transfer steak to cutting board; let rest 10 minutes.
Lightly brush cut sides of rolls with remaining 2 tbsp of the oil. Place rolls, cut side down, on grill rack. Cook 1 - 2 minutes or until golden.
Thinly slice steak across grain on the diagonal. Divide steak among bottom halves of rolls; top each with one-fourth of the onions. Spread cut sides of baguette tops with cheese mixture; place on top of onions. Serve immediately.