Flank Steak with Red Onions

2/3 cup balsamic vinegar 1 tablespoon olive oil 6 cups vertically sliced onion (about 1 1/2 pounds) 1/2 teaspoon salt, divided 1 (1-pound) flank steak,[...]

1 1 1/4-lb. flank steak Salt and pepper 1 tablespoon vegetable oil 3 tablespoons unsalted butter 2 red onions, cut in half lengthwise, thinly sliced crosswise[...]