To make the marinade, mix together the oyster sauce, fish sauce, sugar and sesame oil. Add the beef and refrigerate overnight to marinate. Remove from the fridge 2 hours before cooking.
To make the dressing, pound the lemongrass, chillies, garlic and sugar in a mortar with a pestle to form a fine paste. Mix the fish sauce and lime juice together and taste for balance.
Heat a grill or a barbecue to hot, and cook the beef for about 2 minutes on each side until it forms a good even crust. Remove from the heat and rest in a warm place for 10 minutes. Slice the beef thickly across the grain.
Toss the beef, onion and coriander leaves with some of the dressing. Arrange the lettuce and beef on a serving plate and drizzle with any remaining dressing. Sprinkle with the ground roast rice, give it a good grind of black pepper and top with the shredded Thai basil.