Step 1: Lightly score the flank steak on the top and bottom in a crosshatch pattern, making shallow (1/8-inch) cuts. Generously season the steak with salt and pepper on both sides. Drizzle with sesame oil, rubbing it into the meat.
Step 2: Make the marinade / dressing: Place the garlic, ginger, lemongrass, and cilantro roots (if using) in a large bowl and stir to mix. Sprinkle the sugar and pepper on top and mash well with the back of a spoon. Let stand for 5 minutes. Add the fish sauce and lime juice and whisk to mix. Set aside.
Step 3: Set up a grill for direct grilling and preheat to high. Brush and oil the grill grate.
Step 4: Grill the steak until cooked to taste, 3 to 4 minutes per side for medium-rare, turning with tongs. Transfer the steak to a cutting board and let rest for 1 minute.
Step 5: Meanwhile, arrange the lettuce leaves on a large platter. Top with the cucumber, tomatoes, and onion.
Step 6: Thinly slice the steak on the diagonal and add the hot slices to the bowl with the marinade / sauce. Toss to mix. Using tongs, arrange the steak slices over the salad, then spoon the remaining marinade / sauce on top. Sprinkle with the mint, basil, cilantro, jalapenos, and peanuts, and you’re in business.
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Steven Raichlen’s Project Smoke is a production of Maryland Public Television, Barbacoa, Inc., and Resolution Pictures. © 2017 Barbacoa, Inc.