Combine beef, lemongrass, garlic, and oil in a large bowl. Season with salt and pepper. Toss to coat.
Place noodles in a heatproof bowl. Cover with boiling water. Set aside for 4 minutes or until softened. Drain. Refresh under cold water. Drain. Place noodles in a bowl.
Dressing: Place hoisin sauce, fish sauce, lemon juice, and sugar in a small bowl. Stir until sugar dissolves.
Heat a wok over high heat. Add half the beef mixture. Stir-fry for 1 to 2 minutes or until browned. Transfer to a bowl. Cover to keep warm. Repeat with the remaining beef mixture.
Add a tablespoon of rice bran oil to the wok and cook the chicken strips. Cook for 1 to 2 minutes or until browned and cooked. Transfer to a bowl. Cover to keep warm.
Place lettuce leaves on a platter. Divide noodle mixture between lettuce cups. Top with beef and drizzle with dressing. Sprinkle with fried sweet potato or fried shallots. and remaining mint and coriander. Serve immediately.