Combine the first 7 ingredients in a medium bowl and whisk until smooth. Add the flank steak to a large zip-top bag and pour the marinade over it. Seal the bag, pressing out excess air, and refrigerate for at least 4 hours and up to 24 hours.
Remove the steak from the fridge 20 to 30 minutes before cooking so it is not cold.
Discard the marinade. Pat the steak very dry with paper towels. A dry surface helps it sear better. Mix the rub ingredients and sprinkle evenly over both sides of the steak.
Set a cast iron skillet or other heavy pan over medium high heat for about 3 minutes until very hot. Add the oil and let it shimmer. The pan should sizzle as soon as the steak hits.
Lay the flank steak in the pan and do not move it for 3 to 4 minutes to build a deep brown crust. Flip and cook the second side for another 3 to 4 minutes. If the steak is thinner than 3/4 inch, cook closer to 2 to 3 minutes per side. If thicker, go closer to 5 minutes per side.
For best results, use a meat thermometer. Pull the steak at 125 to 130°F for medium rare. If you do not have a thermometer, check by feel. Press the center gently. Very soft means rare. Soft with a little spring means medium rare. Firmer with less give means medium. Flank steak is best cooked medium rare to medium. Cooking it more than that will make it tough.
Transfer the steak to a cutting board and tent loosely with foil. Rest for 8 to 10 minutes so the juices redistribute.
Look for the long muscle lines running across the steak. Turn the steak and slice thin across those lines, not along them. Slice on a slight diagonal. This step is very important for tender steak.
Notes
Note 1: London broil refers to the cooking method, not the cut of meat. I have tested this method with many cuts, and flank steak is my hands down favorite for the best flavor and most reliable results. A thin flat iron steak is a great second option.Storage: For best results, enjoy this London Broil Recipe fresh. Leftovers should be cooled completely, stored in an airtight container, and used within 3 days. Serve reheated gently or sliced thin and cold.