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1
Combine rice vinegar, water, sugar, and salt in a small saucepan and bring to a boil. Add onions and onion bottom. Cover pot and remove from heat. Allow onions to pickle for at least 1 hour before using. Pickled onions can be stored in a sealed container covered in brine in the refrigerator for at least 2 weeks.
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2
Heat rice in a dry 10-inch skillet over medium-high heat, tossing and stirring constantly until deep golden brown, about 5 minutes. Transfer to a spice grinder and grind into a fine powder. Set aside.
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3
Grind duck through the course plate of a food grinder, or by pulsing in a food processor 1/2-pound at a time with 10 to 12 one second pulses.
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4
Heat oil in a 12-inch skillet over high heat until lightly smoking. Add duck and cook, stirring occasionally, until no longer pink and fat has mostly rendered. Drain fat and return duck to skillet. Add shallot, garlic, ginger, lemongrass, and half of chili. Cook, stirring occasionally, until aromatic, about 1 minute. Add long beans and red onions and continue to cook, stirring constantly, for 1 minute. Add herbs, fish sauce, and lime juice, and toss to combine. Taste for heat and add remaining chili powder if desired.
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5
Place lettuce leaves with cupped side facing up on a serving platter. Spoon larb into lettuce leaves, sprinkle with toasted rice powder, and serve.
