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1
Combine water and sugar in a small saucepan and heat over medium heat until completely dissolved. Transfer to a small bowl, add fish sauce, lime juice, garlic, and chili powder and stir to combine. Transfer half to a small container and reserve until step 3. Add oil to remaining half and whisk to combine. Place flank steak inside a gallon-sized zipper-lock bag with marinade. Press out air, seal bag, and allow meat to marinate, turning occasionally, for at least 1 hour and up to 12.
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2
Remove steak from marinade and pat dry with paper towels. Ignite a large chimney full of coals and wait until they're covered in grey ash. Spread evenly over 1/2 of grate, leaving the other half empty. Put the cooking grate in place, cover, and allow grill to preheat for 5 minutes. Scrape cooking grates clean, then place flank steak over hot side of grill. cook until well charred, about 3 minutes. Flip steak and continue to cook until second side is well charred, about 3 minutes longer. Transfer steak to cooler side of grill, cover, and cook until the center of the steak registers 125u00b0F on an instant-read thermometer for medium-rare, or 135u00b0 for medium, about 5 minutes longer. Transfer to a cutting board, tent with foil, and allow steak to rest for at least 5 minutes. Proceed to step 4.
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3
Heat 2 tablespoons vegetable or canola oil in a large stainless steel or cast iron skillet over high heat until shimmering. Add steak and cook, turning frequently, until an instant-read thermometer registers 125u00b0F for medium-rare or 135&Deg;F for medium, reducing heat as necessary if steak smokes excessively or starts to burn. Transfer to a wire rack set in a rimmed baking sheet and allow to rest 5 to 10 minutes.
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4
Thinly slice beef against the grain and transfer to a large bowl. Add herbs, shallots, bean sprouts, and reserved marinade, and toss to combine. Serve immediately.
