In a small saucepan over high heat, bring the stock to a boil. Add the meat and cook for 1 to 2 minutes, tossing often with a large spoon to break up the meat and cook it to fairly evenly. When the meat is cooked, remove the pan from the heat.
Using a slotted spoon, transfer the meat to a medium bowl, leaving most the of the liquid behind. Stir in the shallot, green onion, cilantro, and most of the mint, reserving a few leaves for a garnish. Add the lime juice, fish sauce, rice powder, chilli, and sugar (if preferred); stir to combine everything well. Taste and adjust seasoning as needed for a pleasing balance of sour, salty, and hot.
Line a serving platter with lettuce leaves and mound meat mixture in the center. Garnish with the cabbage, green bean, and mint. Serve at once.