Rub olive oil on all sides of the London broil. Rough chop fresh rosemary. Sprinkle a generous amount of rosemary and thyme each side. Either use a pepper grinder set on fine or put peppercorns into a Ziploc bag and pound. Put a generous amount of salt and pepper on all sides along with onion powder (just go easy on the onion powder). Let this marinate for at least an hour; overnight would be better.