Fish sauce, a pungent Southeast Asian condiment, gives this easy stir-fry a flavor punch, but you can use reduced-sodium soy sauce instead. Add more or less crushed red pepper to adjust the heat.
Combine lime juice, fish sauce (or soy sauce), 1 Tbsp. oil, and brown sugar in a small bowl. Set aside.
Heat 1 Tbsp. oil in a flat-bottom carbon-steel wok or large skillet over medium-high heat. Add steak; cook, stirring, until no longer pink, about 5 minutes. Transfer to a bowl. Wipe out the pan. Add the remaining 1 Tbsp. oil to the pan. Add beans and bell pepper; cook, stirring frequently, until the beans are tender-crisp and the bell pepper and beans are beginning to char, 7 to 10 minutes. Add garlic and crushed red pepper; cook, stirring, until the garlic is fragrant, 30 seconds to 1 minute more. Remove from heat. Add the reserved lime juice mixture and steak, cilantro, and peanuts. Stir to combine.
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