Heat half the oil in a wok over high heat until just smoking. Add lamb and cook, stirring with a spatula to break up any lumps, until lamb changes colour and begins to brown. Remove lamb, set aside.
Return wok to the heat, add remaining oil, garlic, chilli and lemongrass. Cook for 2 minutes or until aromatic. Return lamb mince to the wok. Stir to combine well. Add fish sauce and sugar, stir to combine.
Remove from heat. Add the eschallots, green onions, mint and coriander. Stir to combine. Season with lime juice and fold the bean sprouts through.
Place lettuce on serving plates. Spoon lamb mixture among serving bowls. Top with toasted rice. Serve immediately.
