2 pints grape tomatoes
6 scallions, white and green parts separated and cut into 1-inch pieces
2 tablespoons olive oil
Coarse salt and ground pepper
1 cup yellow cornmeal
1 tablespoon butter
1/4 cup finely grated Parmesan
1 1/2 pounds skirt steak (cut into 2 or more pieces, if necessary, to fit in skillet)
1 cup balsamic vinegar
Preheat oven to 400 degrees. In a large saucepan, set 4 cups water to boil. On a large rimmed baking sheet, toss tomatoes with scallion whites and 1 tablespoon oil; season with salt and pepper. Roast until tomatoes are tender and some skins have split, 12 to 15 minutes; toss with scallion greens.
Meanwhile, add 1 teaspoon salt to boiling water; gradually whisk in cornmeal. Simmer very gently over low heat, whisking occasionally, until polenta is thickened and cooked through, about 10 minutes. Whisk in butter and Parmesan, and keep warm over very low heat (whisk in some water just before serving if polenta becomes too thick).
Heat remaining tablespoon oil in a large skillet over high. Season steak with salt and pepper; add to skillet. Cook, turning once, 6 to 8 minutes total for medium-rare. Transfer to a plate, cover loosely with aluminum foil, and let rest, 5 to 10 minutes (reserve skillet).
Add vinegar to skillet, and boil over high until reduced to 1/2 cup, 5 to 7 minutes; stir in any juices from resting steak. Slice steak, and serve with vinegar sauce, polenta, and tomatoes.