London broil is traditionally a method of cooking steak and not a steak unto itself. This method originally involved pan searing and oven cooking, with no grilling at all. The terminology of "London broil" has evolved to describe thick top round steaks ideal for pan cooking. Traditional London broil recipes rarely call for any kind of marinade, are lightly seasoned and are relatively simple to cook for beginners in the kitchen.
Remove the steak from the refrigerator and let it sit in a clean place for two hours to reach room temperature.
Use a meat tenderizer to pound the steak flat, if necessary, to even out the thickness. Trim excessive fat or additional tissue, as desired.
Set the stove to medium high. Rub dry mustard powder onto both sides of the steak, and add salt and pepper to season. Use the butter to rub onto the steak to add flavor and help lubricate it while cooking in the skillet.
Heat a cast-iron skillet on the stove, and place the seasoned steak in the skillet. If your steak is 1 inch thick or less, sear it for two to three minutes on each side, checking to see that it is browned before flipping it. If your steak is thicker, it may need extra time.
Leave the steak in the pan for a few minutes if it is 1 inch thick or less to finish cooking to a medium-rare cook. If it is thicker, you can place the skillet in the oven at 325 degrees Fahrenheit for 15 minutes to finish the cooking. Slice it thinly to serve.