Summarize:

As with most other cuts of meat, flank steak is at its most tender and flavorful not right after it comes off the grill, but after it's had a little time to rest and cool. If the meat is cut immediately after it's removed from the grill, you'll notice its internal juices spill onto the plate. These juices are responsible for the meat's flavor and texture, so doing this can make your meat lest tender and tasty. However, if you give your meat a short "break" before cutting into it, it has a chance to re-absorb these juices, retaining much of its moisture and flavor. To "rest" your meat, place it on a clean plate or platter under tented aluminum foil and let it sit for 10-15 minutes. The foil will help the meat retain its heat as it rests. Flank steak is made up of long, thin muscle fibers you can see them if you look for small, faint lines running the length of the meat. If left intact, these fibers can be tough to chew. However, by cutting the meat diagonally, perpendicular to the grain of the meat, the fibers are severed, making the slices of meat much more tender. For this reason, London Broils are traditionally served as narrow diagonal slices made against the grain of the meat. Divide your London Broil into a portions consisting of a few slices each. If you wish, you can now season your portions individually. A light sprinkling of salt and fresh-ground black pepper almost always pairs well with London Broil, but other seasonings can make great choices as well. Below are just a few seasoning ideas: Pre-made seasoning mixes (Johnny's, etc.) Rosemary, thyme, and roasted garlic Chili powder Paprika Grilled/fried onions Congratulations! Your delicious London Broil is all ready to eat. Try serving this classic dish with roasted vegetables or sandwiching it between a few pieces of bread for classic flavor combinations.
Allow the meat to "rest" before cutting it. Slice against the grain. Season as desired. Enjoy!