Place freezer zip lock baggy inside a container (to easily add all the contents of the marinade). Add soy sauce, minced onion, brown sugar, sesame oil, toasted* sesame seeds, ginger, garlic, and red pepper. Mix well, then add in the London Broil. Seal the bag and knead all the ingredients together.
Marinade for 12-24 hours.
Once your ready to cook. Preheat grill to a medium-high heat. Clean prior to grilling steak.
Carefully remove steak from bag, and reserve marinade.
Grill for 10-16 minutes so that the steak is medium-rare to medium (depending on how you like your steak).
While steak is cooking add remaining marinade in a sauce pot. Cook on medium heat until the marinade has reduced to half. Remove from heat once it's done reducing.
Let the steak sit for at least 5 minutes. Once ready, slice the steak against the grain in very thin slices. After the steak is cut drizzle the reduced marinade on top. Serve and enjoy.