1. Trim off the external sinew of the lamb leg. With a knife, separate the muscles by following along where the muscles naturally separate and slice against the grain into thin strips, removing any sinew as you work.
2. Transfer lamb to a bowl and use your hands to mix the meat with the cornstarch, pepper, salt, and 1 tablespoon of the oil. Let marinate at room temperature while you prepare the remaining ingredients.
3. In a dry wok or skillet, toast the cumin seeds and mustard seeds until the cumin is fragrant and the mustard seeds start to pop, no more than 30 seconds. Transfer to a mortar and use a pestle (or a coffee grinder used for grinding spices) to pulverize to a coarse powder.
4. In a small bowl, stir together the soy sauces, 1 teaspoon of the rice vinegar, and the sugar.
5. Heat the wok over high heat. Add 1 tablespoon of the oil and swirl to coat the sides. Stir-fry the lamb until the pieces have separated and are lightly browned but not fully cooked, about 2 minutes. Transfer to a plate.
6. Heat the remaining 1 tablespoon of oil in the wok. Add the garlic and jalapeños and cook briefly, about 15 seconds. Add the mustard-cumin blend, chiles, and chile flakes and stir-fry briefly. Pour in the sauce mixture, increase the heat to high, and let the sauce come to a boil.
7. Add the lamb and onions and cook, stirring often, until the lamb is cooked through but the onions are still slightly crisp, 2 to 3 minutes. Add the basil and the remaining 1 teaspoon vinegar. Taste, adding more soy sauce or vinegar if desired. Garnish with more basil.

Reprinted with permission from Burma Superstar [2], copyright © 2017 by Desmond Tan and Kate Leahy. Published by Ten Speed Press, an imprint of Penguin Random House LLC.Photographs copyright © 2017 by John Lee