Food.com offers a recipe for grilled flat-iron steak, and MarthaStewart.com features a recipe for seared flat-iron steak with wine sauce. You can cook either recipe in a cast-iron skillet or heavy-bottomed skillet or on a grill.
To make Martha Stewart's marinated flat-iron steak, combine 1/4 cup of olive oil, 2 crushed cloves of garlic, 1 teaspoon of Italian seasoning and 2 tablespoons of balsamic vinegar. Pour the prepared mixture over 1 1/4 to 1 1/2 pounds of flat-iron steak in a sealed plastic bag and marinate for one to eight hours. After marinating the meat, allow it to sit at room temperature for at least an hour before cooking. Cook the meat on a grill for five minutes on one side, then turn it over and cook it one the other side for four minutes on a slightly lower heat. Slice and serve immediately.
To make Food.com's flat-iron steak, season 4 6-ounce pieces of flat-iron steak with salt and pepper, and then sear each side of the meat for four to eight minutes, depending on preference, and cover the meat with a reduction of 1 cup of dry red wine and 2 tablespoons of butter. After cooking, place the meat on a plate and cover with foil. Simmer the wine on medium-high with the steak drippings before removing from heat before adding chopped butter and stirring slowly. Add seasoning to the sauce before drizzling over the cooked meat.