Wrap roast in plastic wrap and place in freezer for 30 minutes. Take beef out of plastic wrap and slice into 1/8-inch thick pieces.
Using a food processor or blender puree lemongrass, shallots, fish sauce, sugar, sesame oil, peanut oil, salt, garlic, and chiles until smooth. Pour mixture into a large resealable plastic bag; add meat; seal and chill for 2 to 4 hours.
Take beef out of bag and lay on a cooking spray coated grill rack. Cook 1 minute per side or until preferred degree of doneness.