For the marinade, whisk together the pineapple juice, soy sauce, Worcestershire sauce, garlic, red pepper flakes, and dried minced onion.
Place the meat into a large storage bag or bowl and pour the marinade over the meat. Toss the meat in the marinade to coat and distribute the marinade evenly over the meat.
Marinate the beef for at least 2 hours and up to 24 hours. I find 8-12 hours best.
When ready to prepare, preheat the broiler on your oven to high heat and adjust an oven rack to be about 4-5 inches from the flame.
Remove the meat from the marinade and place it onto a sheet pan and discard the marinade.
Place the sheet pan into the oven and broil for 5 minutes. Remove from the oven and flip the meat and return to the oven and broil for 3-4 minutes or until an internal temperature of 130 degrees F for medium-rare.
Remove the meat from the oven and allow to rest for 10 minutes--it will rise in temperature to about 135 degrees. Then cut against the grain (perpendicular to the parallel lines of muscle fiber running down the meat) into very thin slices.
Cut of Beef: London Broil is not a cut of beef, but instead a preparation method using either flank or round steak. However, it is often sold labeled London Broil. Temperature of the Meat: London Broil is best served medium-rare, which is an internal temperature of 130-135 degrees. This is best determined by inserting an oven-safe digital thermometer into the center of the steak and monitoring the temperature. Cook to 140-145 degrees for medium doneness, 150-155 for medium-well, or 160-165 degrees for well done. Leftover London Broil can be stored in a sealed container for 3 days. Reheat slices in a dry skillet quickly until just warm, or broil unsliced London Broil for 2 minutes per side just to warm through. Control the heat by omitting or decreasing the amount of crushed red pepper flakes in the marinade.