Preparation

• In a large nonreactive bowl, combine the steak, lemongrass, 4 tsp. of the fish sauce, 1 tsp. of the sugar, and 2 Tbs. of the garlic. Stir well to coat the meat. Cover the bowl with plastic wrap and let marinate for 20 minutes. Meanwhile, arrange the cucumbers, lettuce, carrots, cilantro, and mint on a large serving platter, leaving room for the steak. Add the noodles to the platter. Put the peanuts in a bowl.
• In a small bowl, combine 1/4 cup water with the remaining 2 Tbs. sugar; stir to dissolve. Add the lime juice, the remaining 2 Tbs. fish sauce, the remaining 1/2 tsp. garlic, and chile, and stir to combine.
• Drain the steak from the marinade, discard the marinade, and pat the steak dry. Heat a large, heavy nonstick skillet over high heat. Add the oil and swirl to coat the pan. When hot, add half of the steak in an even layer in the pan. Cook, undisturbed, for 1 minute. Stir and continue cooking, stirring occasionally, until the steak is browned and the juices have almost evaporated, 1 to 2 minutes longer. Transfer the steak to the serving platter and repeat with the remaining steak, adding more oil as needed. Serve immediately, with the sauce on the side.