Freezing the beef makes slicing it easy. Reserve a quarter of the cooked beef for Vietnamese Summer Rolls.
Cover roast with plastic wrap; freeze 30 minutes. Remove beef from freezer, and cut beef horizontally into 1/8-inch-thick slices.
Combine lemongrass and remaining ingredients except cooking spray in a food processor; process until smooth (about 1 minute). Combine beef and lemongrass mixture in a large zip-top plastic bag. Seal and marinate in refrigerator 2 to 4 hours.
Remove beef from bag; discard marinade. Place beef on grill rack coated with cooking spray; grill 1 minute on each side or until desired degree of doneness.
Note: Store cooked beef in refrigerator for up to 2 days.