To finish the salad:
1 head Boston or Bibb lettuce, separated into leaves, rinsed, and spun dry
1 hothouse or English cucumber, very thinly sliced
1 small sweet onion, very thinly sliced
12 cherry tomatoes, halved
12 fresh mint leaves (optional)
¼ cup fresh cilantro leaves
¼ cup coarsely chopped dry-roasted peanuts
1. Lightly score the flank steak in a crosshatch pattern, making the cuts ¼-inch deep. Place the meat in a glass baking dish. Combine soy sauce, fish sauce, sugar, garlic, and ginger in a mixing bowl; whisk until sugar dissolves. Pour mixture over steak and let marinate, covered, in the refrigerator for at least two hours or as long as eight hours, turning several times.
2. Preheat grill to medium-high.
3. To make dressing, grind the garlic, chilis, and sugar to a paste using a mortar and pestle. Add fish sauce and lemon juice. (If you don’t have a mortar and pestle, you can puree in a blender or a small food processor.)
4. To prepare the salad, cover a platter with the lettuce leaves and arrange cucumber, onion, cherry tomatoes, and mint leaves (if using) on top.
5. When steak is marinated, oil the grill grate, then place steak on the hot grate and grill until cooked to taste (four to six minutes per side for medium-rare), using tongs to turn. Transfer steak to a cutting board and let cool.
6. This salad can be served warm or at room temperature. Thinly slice the steak across the grain on the diagonal. Spoon dressing over the salad and arrange beef slices on top. Sprinkle with cilantro and roasted peanuts, then serve.
