Flat iron steaks go by several names, but most often top blade steaks or top chuck steaks, as they're from the cow's chuck section. Chuck refers to cuts from the neck and shoulder, which are cost-effective and flavorful, but on the tougher and fattier side. Because baking uses dry heat, it's not the best choice for less tender cuts like those from the chuck. But flat iron steaks are generally just tender enough to bake if you marinate them first. Most marinades add flavor, but don't tenderize, so choose a marinade that will make the steak tender.
Pour enough pineapple juice, papaya juice or buttermilk into a glass, ceramic or other dish to cover the flat iron steak. These liquids have enzymatic properties that tenderize -- not just flavor -- meat. Add salt and pepper to taste. Season the marinade with herbs and spices, if you like, such as fresh or dried rosemary, cilantro, dill or cayenne pepper. Do not use a metal dish for this marinade.
Place the flat iron steak in the marinade and cover the dish with its lid or plastic wrap. Put it into the refrigerator. Soak the beef in pineapple or papaya juice marinades for about an hour. If it's buttermilk marinade, leave it in there two to three hours. Leaving the meat in the marinade for too long can make it mushy.
Heat the oven to 350 degrees Fahrenheit about 20 minutes before cooking the flat iron steak.
Cover the bottom of a roasting pan with aluminum foil. Spray the pan's rack with nonstick spray and put it in the pan. The foil will make the pan easier to clean.
Take the beef out of the marinade and let the excess drip off into the dish. Place the flat iron steak on the rack of the roasting pan and put it into the oven. Throw the marinade out.
Bake the flat iron steak to the desired level of doneness, checking it with a food thermometer. Cook it for about 10 minutes to 120 F for rare; about 12 minutes to 130 F for medium-rare; about 15 minutes to 140 F for medium; about 20 minutes to 150 F for medium-well; or about 23 minutes to 160 F for well done.
Take the flat iron steak out of the roasting pan and put in on a plate. Let it rest for five to 10 minutes. This allows the moisture that seeps out of the muscle fibers during cooking to be reabsorbed, making the steak juicier.